In the context of e-Learning, interactivity is defined as "function of input required by the learner while responding to the computer, the analysis of those responses by the computer, and the nature of the action by the computer." 
There are two variables describing the interactivity of a given lesson: technology affordance and user freedom. Technology affordance, also known as manual operators, is the richness of the communication between the student and the instructor usually expressed in terms of the input and output channels. User freedom, on the other hand, is a function of the degree of freedom the learning system grants the student in influencing the presentation of the lesson.
Technology affordance in the e-Learning is a function of the capabilities of available computer I/O devices, having a tendency to evolve as new input and output devices become obtainable. There are five levels of technology affordance interactivity:
Immersion is full sensory immersion commonly called virtual reality. Video games are the closest approximation we have to Immersion today.
Voice. The same as Text except emotion and body language are used as communication channels. A phone conversation is an example of voice level interactivity.
Text. Allowing the student to communicate through arbitrary textual input and output. A Google search box or a discussion board interaction are examples of text level interactivity.
Menu Select. Providing the learner the capability to "selecting an answer among multiple choices questions [or] pulling down a menu and selecting a menu item". Clicking on a hyperlink in a web page is an example of menu-select interactivity.
Toggle Select. The ability to "click a button or press a key". A pinball machine, for example, exhibits toggle-select interactivity.
User freedom is "a measure of a medium's potential ability to let the user exert an influence on the content and/or form of mediated communications."  There are nine levels of user freedom interactivity:
Create. Create level interactivity enables the student to explore and experiment with the subject matter in an unconstrained way. The system provides an environment that simulates an external reality and the student, through interaction with the environment, develops his own understanding of the external reality. Requirements include representation of an external reality, the ability of the student to interact with the reality, and evaluation of the solution. An e-learning example would be a flight simulator emulates the behavior of the airplane allowing a student pilot the ability to sharpen his skills and better understand the airplane. A traditional classroom example would be a chemistry lab where students can experiment with different chemicals in an effort to better understand their properties and how they interact.
Construct. Construct level interactivity allows the student to build a new tool from a provided set of tools that will then be used to perform a task. Requirements include representation of a collection of tools, the ability of the student to control the tools, a mechanism to combine tools, and evaluation of the students' solution. An e-learning example would be a statistics student is asked to determine the relationship between two groups in a population using the statistical tool SPSS. A traditional classroom example would be: given a ruler and a collection of sticks, a geometry student is asked to create a right triangle through the use of the Pythagorean theorem.
Use. Use level interactivity allows the student to perform a task with a provided tool or set of tools. Requirements include representation of a tool with which the user may solve a problem and evaluation of the solution. An e-learning example would be students being asked to find the mean grade from a list in a spreadsheet using the =average() function. A traditional classroom example would be: in a biology class, each student is given a scale and the task of finding the average weight of a population of fish.
Do. Do level interactivity allows the student to respond to a prompt for input and for the system to evaluate the response. Requirements include the ability to ask the student a question and evaluate the quality of the response against a pre-specified criteria. An e-learning example would be a student taking an online quiz. Feedback is given in the form of his answers being compared against the instructor-provided correct answer. A traditional example would be a student being asked a question in class and the instructor evaluating the his response.
Request. Request level interactivity gives the student the ability to request content not specified on a pre-defined list. The system needs to translate the student's request to a standard format and then to query a knowledge base to find the proper response. An e-learning example would be a student looking up a fact on the internet using a search engine to help clarify a point of the lesson. A traditional classroom example would be a student asking the instructor to clarify a point in class.
Select. Select level interactivity gives the student the ability to choose which content is to be viewed from a pre-specified list of choices. Three things must occur: subdivision of the lesson content into sections, presentation of the section list to the student, and the ability to jump to a given section from any point in the lesson. An e-learning example would be a political science teacher posting on the course site a collection of articles related to the day's discussion. They can be read in any order or not at all. A traditional example would be a physics professor gives the class the choice of practice problems to work out in class.
Sequence. Sequence level interactivity gives the student the ability to change the order in which content is presented. A constraint exists that all the content must be eventually viewed. It is required for content to be divided into labeled sections, each of which must be randomly accessible. A mechanism must be presented to the user to alter the default ordering of the content. Finally, a mechanism must exist to enforce that all the content will eventually viewed. An e-learning example would be songs on a playlist are reordered but, after enough play time, all the songs are eventually heard. A traditional classroom example would be two students take courses for the same major in different order. Nevertheless, the requirements for the major remain the same for both.
Pace. Pace level interactivity gives the student the ability to pause, speed up, and rewind the presentation of the instructional material. There student must be able to perform any of the following operations: pause, rewind, fast forward, or skip to a given point in the timeline. Online, this could be watching a QuickTime movie, podcast or a clip on YouTube where the student can pause, fast forward, or rewind the movie. In a traditional lesson, an instructor could allow the students to pause a lecture so he could catch up on their notes or digest the content. The instructor would also need to be responsive to requests to speed up or repeat parts of the presentation of the content.
Watch. Watch level interactivity gives the student no ability to control the presentation of the content. The system makes no provision to accept input from the user. This could be viewing a flash animation that illustrates a given concept where the student has no capacity to influence the presentation of the animation or listening to a lecture in a crowded lecture hall where the speaker does not field questions from the audience. Watch level interactivity requires complete absence of interactive controls; the system maintains complete control.
^Chen, M. (1995), "A methodology for characterizing computer-based learning environments" in Instructional Science
^Helfrich, J.; Moulton, S. (2009), "Leveraging interactivity to increase e-learning effectiveness" in Proceedings of World Conference on E-Learning in Corporate, Government, Healthcare, and Higher Education 2009
^Jonassen, D. (1988), Instructional Designs for Microcomputer Courseware