An art song is a vocal musiccomposition, usually written for one voice with piano accompaniment, and usually in the classical tradition. By extension, the term "art song" is used to refer to the genre of such songs. An art song is most often a musical setting of an independent poem or text, "intended for the concert repertory" "as part of a recital or other relatively formal social occasion".
While many pieces of vocal music are easily recognized as art songs, other are more difficult to categorize. For example, a wordless vocalise written by a classical composer is sometimes considered an art song. and sometimes not 
Other factors help define art songs:
Songs that are part of a staged work (such as an opera or a musical) are not usually considered art songs However, some Baroque arias that "appear with great frequency in recital performance" are now included in the art song repertoire.
Songs with instruments besides piano and/or other singers are referred to as "vocal chamber music", and are usually not considered art songs.
Songs originally written for voice and orchestra are called "orchestral songs" and are not usually considered art songs, unless their original version was for solo voice and piano.
There is no agreement regarding sacred songs. Many song settings of biblical or sacred texts were composed for the concert stage and not for religious services; these are widely known as art songs (for example, the Vier ernste Gesänge by Johannes Brahms). Others sacred songs may or may not be considered art songs.
A group of art songs composed to be performed in a group to form a narrative or dramatic whole is called a song cycle.
Languages and nationalities
Art songs have been composed in many languages, and are known by several names. The German tradition of art song composition is perhaps the most prominent one; it is known as Lieder. In France, the term Mélodie distinguishes art songs from other French vocal pieces referred to as chansons. The Spanish Canción and the Italian Canzone refer to songs generally and not specifically to art songs.
Art song formal design
The composer's musical language and interpretation of the text often dictate the formal design of an art song. If all of the poem's verses are sung to the same music, the song is strophic. Arrangements of folk songs are often strophic, and "there are exceptional cases in which the musical repetition provides dramatic irony for the changing text, or where an almost hypnotic monotony is desired." Several of the songs in Schubert's Die schöne Müllerin are good examples of this. If the vocal melody remains the same but the accompaniment changes under it for each verse, the piece is called a "modified strophic" song.
In contrast, songs in which "each section of the text receives fresh music" are called through-composed. Some through-composed works have some repetition of musical material in them.
Many art songs use some version of the ABA form (also known as "song form"), with a beginning musical section, a contrasting middle section, and a return to the first section's music.
Art song performance and performers
Performance of art songs in recital requires some special skills for both the singer and pianist. The degree of intimacy "seldom equaled in other kinds of music" requires that the two performers "communicate to the audience the most subtle and evanescent emotions as expressed in the poem and music." The two performers must agree on all aspects of the performance to create a unified partnership, making art song performance one of the "most sensitive type(s) of collaboration".
Villamil, Victoria Etnier (1993), A Singer's Guide to the American Art Song (2004 paperback ed.), Lanham, Maryland: Scarecrow Press, ISBN0-8108-5217-9
Emmons, Shirlee, and Stanley Sonntag (1979), The Art of the Song Recital (paperback ed.), New York: Schirmer Books, ISBN0-02-870530-0
Hall, James Husst (1953), The Art Song, Norman, Oklahoma: University of Oklahoma Press
Ivey, Donald (1970), Song: Anatomy, Imagery, and Styles, New York: The Free Press, ISBN0-8108-5217-9
Soumagnac, Myriam (1997). "La Mélodie italienne au début du XXe siècle", in Festschrift volume, Échoes de France et d'Ialie: liber amicorum Yves Gérard (jointly ed. by Marie-Claire Mussat, Jean Mongrédien & Jean-Michel Nectoux). Buchet-Chastel. p. 381-386.
Walter, Wolfgang (2005), Lied-Bibliographie (Song Bibliography): Reference to Literature on the Art Song, Frankfurt am Main: Peter Lang, ISBN08204-7319-7
Whitton, Kenneth (1984), Lieder: An Introdution to German Song, London: Julia MacRae, ISBN0-531-09759-5