Integrated Marketing Communication (IMC) is a term that emerged in the late 20th century regarding application of consistent brand messaging across myriad marketing channels. The term has varying definitions and the differences can play a part in how IMC is viewed and used:
The first definition for integrated marketing communication came from the American Association of Advertising Agencies (also 4A's) in 1989, defining IMC as "an approach to achieving the objectives of a marketing campaign through a well-coordinated use of different promotional methods that are intended to reinforce each other." The 4A's definition of IMC recognizes the strategic roles of various communication disciplines (advertising, public relations, sales promotions, etc.) to provide clarity, consistency, and increased impact when combined within a comprehensive communications plan. Basically, it is the application of consistent brand messaging across both traditional and non-traditional marketing channels.
The Journal of Integrated Marketing Communication from the Medill School of Journalism at Northwestern University refers to IMC as "a strategic marketing process specifically designed to ensure that all messaging and communication strategies are unified across all channels and are centered around the customer." IMC is used practically to allow one medium's weakness to be offset by another medium's strength, with elements synergized to support each other and create greater impact.
A more contemporary definition states, "True IMC is the development of marketing strategies and creative campaigns that weave together multiple marketing disciplines (paid advertising, public relations, promotion, owned assets, and social media) that are selected and then executed to suit the particular goals of the brand." Instead of simply using various media to help tell a brand's overall story, with IMC the marketing leverages each communication channel's intrinsic strengths to achieve a greater impact together than each channel could achieve individually. It requires the marketer to understand each medium's limitation, including the audience's ability/willingness to absorb messaging from that medium. This understanding is integrated into a campaign's strategic plan from the very beginning of planning - so that the brand no longer simply speaks with consistency, but speaks with planned efficacy. This concept inherently provides added benefits that include: a singular/synchronized brand voice and experience, cost efficiencies generated through creativity and production, and opportunities for added value and bonus.
Online advertising: mobile advertising, email ads, banner ads, search engine result pages, blogs, newsletters, online classified ads, media ads
Direct marketing: direct mail, telemarketing, catalogs, shopping channels, internet sales, emails, text messaging, websites, online display ads, fliers, catalog distribution, promotional letters, outdoor advertising, telemarketing, coupons, direct mail, direct selling, grassroots/community marketing, mobile
When these diverse aspects of business and marketing are weaved together properly an effective campaign can be achieved. Effective campaigns are demonstrated on the Integrated Brands showcase which recognizes brands that are innovative, strategic and successfully growing their sales.  By effectively leveraging each communication channel greater impact can be achieved together than achieved individually. 
First defined by the American Association of Advertising Agencies in 1989, IMC was developed mainly to address the need for businesses to offer clients more than just standard advertising. The 4As originally coined the term the "new advertising," however this title did not appropriately incorporate many other aspects included in the term "IMC" - most notably, those beyond traditional advertising process aside from simply advertising.
Overall, an influx of new marketplace trends in the late 20th century spurred organizations to shift from the standard advertising approach to the IMC approach:
Decreasing message impact and credibility: The growing number of commercial messaging made it increasingly more difficult for a single message to have a noteworthy effect.
Decreasing costs of databases: The cost of storing and retrieving names, addresses and information from databases significantly declined. This decline allowed marketers to reach consumers more effectively.
Increasing client expertise: Clients of marketing and public relations firms became more educated regarding advertising policies, procedures and tactics. Clients began to realize that television advertising was not the only way to reach consumers.
Increasing mergers and acquisitions of agencies: Many top public relations firms and advertising agencies became partners or partnered with other communication firms. These mergers allowed for more creativity, and the expansion of communication from only advertising, to other disciplines such as event planning and promotion.
Increasing global marketing: There was a rapid influx in advertising competition from foreign countries. Companies quickly realized that even if they did not conduct business outside their own country, they were now competing in global marketing.
Increasing media and audience fragmentation: With the exception of the decline of newspapers, media outlets, such as magazines and television stations, increased dramatically from 1980 to 1990. Additionally, companies could use new technologies and computers to target specialized audiences based on factors such as ethnic background or place of residence.
Increasing number of overall products: Manufacturers flooded retailers with a plethora of new products, many of which were identical to products that already existed. Therefore, a unique marketing and branding approach was crucial to attract customer attention and increase sales.
Model & Stages
Similar to the definition of IMC, models of the IMC approach vary according to the source cited. Frequently, models stress the importance of blending various marketing tools to maximize the customer experience and value. IMC models also often emphasize the lack of a specific hierarchy of importance in the IMC stages: all components of the model play an equally important role and a company may or may not choose to immediately implement any or all of the integration strategies.
Stage one: Awareness: Knowledge of changing business, social, political and cultural trends creates the demand for a new approach to marketing.
Stage two: Image integration: Having a consistent image, message and feel to an organization is crucial. The image reflected in corporate symbols serves as an important element for maintaining consistency in an organization.
Stage three: Functional integration: Analyzing the strengths and weaknesses of each functional area of communication (public relations, event planning, media, etc.), and determining how the different areas can come together to create an effective campaign.
Stage four: Coordinated integration: All communication functions become equal in their potential to influence company marketing efforts and many integration barriers cease to exist.
Stage five: Consumer based integration: The elements of the different communication functions begin to work together. The campaign begins to achieve greater marketing effectiveness because only fully targeted consumers are exposed to the strongest and most effective forms of marketing and media.
Stage six: Stakeholder based integration: Beyond just the customer, there are stakeholders who depend on the positive outcome of marketing campaigns. Companies carefully identify stakeholders, determining who can or will be affected by the success or failure of their campaign. At this stage of integration, the IMC expands from a sales-driven goal to a broader communication goal. This means that a company needs to enhance past marketing efforts through clear, correct and open communication between all parties involved.
Stage seven: Relationship management integration: A fully integrated communication strategy exists that brings customers and stakeholders into direct contact with the business.
Schultz and Schultz identified four levels of IMC through which organizations appear to progress. These stages are not discrete, finite stages with well-defined boundaries Ultimately, however, to be truly integrated an organization needs to demonstrate competency in the activities and requirements of each of the four levels. 
Level 1: Tactical Coordination and Marketing Communications Initial IMC focus is on the tactical coordination of diverse marketing such as advertising, promotion, direct response, public relations, and special events. This level focuses on delivering “one sight, one sound” via marketing communication. 
Level 2: Redefining the Scope of Marketing Communication The organization begins to examine communications from the customer’s point of view. Marketing communication begins to give consideration to all sources of brand and company contact a customer has with the product or service. Management broadens the scope of communication activities to encompass and coordinate internal marketing employees, suppliers, and other business partners and align with the existing external communication programs. 
Level 3: Application of Information Technology An organization’s application of empirical data using information technology to provide a basis identity, value, and monitor the impact of integrated internal and external communication programs to key customer segments over time. 
Level 4: Financial and Strategic Integration The emphasis shifts to using the skills and data generated in the earlier stages to drive corporate strategic planning using customer information and insights. Organizations re-evaluate their financial information infrastructure. 
The Growing Importance of Integrated Marketing Communications
Integration has become an essential concept in marketing because technological advances have changed how business stakeholders interact. Marketing theory that was established during the discipline’s formative years has been overtaken by the complexities of real-time, multimodal, multi directional communication. 
A few examples help illustrate the growing importance of integration:
Search marketing: When someone is considering buying a product or service they will often conduct an online search. What they find, on Google and other search engines, as well as information from news sites, review sites, directories, videos and place-based searches, are presented together, so like it or not, there is a level of integration. The online experience will affect their attitudes towards a brand and their behavior. Marketers therefore need to concern themselves with making sure their brand is found ahead of competitors' and then ensuring their audience has a positive and helpful experience.
Accessibility & convenience: Consumers expect information and services that relate to a brand to be conveniently accessible via its website. For instance when a consumer visits Virgin.com they are able to book a flight, manage their money, top up their mobile phone plan or find up-to-date news about the company. 
Aggregation of information and services: The traditional demarcation between a company, its suppliers and customers has become confused. For instance the Apple iTunes app store aggregates software and information from app makers, along with reviews provided by consumers.  Product promotion, delivery, service and information from many different sources are seamlessly presented together.
Social media: Traditionally businesses were largely in control of their brand communications. Now brand communications are multidirectional as consumers can easily share, comment and create content. Brands can use this to their advantage by creating appealing content. For instance Unilever’s campaign for Dove, The Dove Real Beauty Sketches went viral with over 54 million views on YouTube. 
Growth of mobile: The growing penetration of smartphones with fast internet connectivity means that marketers need to take into consideration integration between the online experience and place-based experiences. For instance when a consumer downloads the Target app they are able to receive coupons to their mobile phone and redeem them at the checkout by presenting the coupon barcode to the cashier. 
Duncan, Tom and Clarke Caywood."The Concept, Process and Evolution of Integrated Marketing Communication." Integrated Communication: Synergy of Persuasive Voices. Ed. Esther Thorson and Jeri Moore. 1st ed. New York: Psychology Press, 1996. 13-34.
Schultz, D. E., & Schultz, H. F. (1998). Transitioning marketing communication into the twenty-first century. Journal of Marketing Communications, 4(1), 9-26.
Beakbane, Tom, "Four reasons why marketers should care about integration," Integrated Brands, June 1, 2013.