An art movement is a tendency or style in art with a specific common philosophy or goal, followed by a group of artists during a restricted period of time, (usually a few months, years or decades) or, at least, with the heyday of the movement defined within a number of years.
Art movements were especially important in modern art, when each consecutive movement was considered as a new avant-garde.
Concept
According to theories associated with modernism and the concept of postmodernism, art movements are especially important during the period of time corresponding to modern art.
The period of time called "modern art" is posited to have changed approximately halfway through the 20th century and art made afterward is generally called contemporary art.
Postmodernism in visual art begins and functions as a parallel to late modernism and refers to that period after the "modern" period called contemporary art.