Minimalism

New York, 1960s
Though never a self-proclaimed movement, Minimalism refers to painting or sculpture made with an extreme economy of means and reduced to the essentials of geometric abstraction. It applies to sculptural works by such artists as Carl Andre, Dan Flavin, Donald Judd, Ellsworth Kelly, John McCracken, Robert Morris, Richard Serra, Tony Smith, and Anne Truitt; to the shaped and striped canvases of Frank Stella; and to paintings by Jo Baer, Ellsworth Kelly, Robert Mangold, Brice Marden, Agnes Martin, and Robert Ryman. Minimalist art is generally characterized by precise, hard-edged, unitary geometric forms; rigid planes of color—usually cool hues or commercially mixed colors, or sometimes just a single color; nonhierarchical, mathematically regular compositions, often based on a grid; the reduction to pure self-referential form, emptied of all external references; and an anonymous surface appearance, without any gestural inflection. As a result of these formal attributes, this art has also been referred to as ABC art, Cool art, Imageless Pop, Literalist art, Object art, and Primary Structure art. Minimalist art shares Pop art’s rejection of the artistic subjectivity and heroic gesture of Abstract Expressionism. In Minimal art what is important is the phenomenological basis of the viewer’s experience, how he or she perceives the internal relationships. 

Post-Minimalism

U.S., late 1960s
Coined by the art historian and critic Robert Pincus-Witten, Post-Minimalism refers to a general reaction by artists in America beginning in the late 1960s against Minimalism and its insistence on closed, geometric forms. These dissenting artists eschewed the impersonal object for more open forms. Rather than adhere to pure formalism, Post-Minimalist artists often made explicit the psychical and physical processes involved in the actualization of art and often reflected personal and social concerns in their works. In his 1987 book Postminimalism to Maximalism: American Art, 1966–1986, Pincus-Witten describes its progression as threefold: pictorial/sculptural, epistemological, and ontological (e.g., conceptual theater/body art). U.S.; late 1960s.

The first, developing circa 1968, emphasized the manufacturing of art and the use of unconventional materials, frequently manifesting a newfound consideration of themes and media previously deemed too feminine, or "soft," according to the Minimalist canon, as can be seen in Eva Hesse's rope and latex pieces and Barry Le Va s scatter installations. The second, beginning circa 1970, reassessed the applicability of theoretical constructs to art production, as evidenced in Sol LeWitt's wall drawings, which exist as descriptions until they are realized by a second party. The third, beginning around 1968, involved the physical presentation of concepts and intentions via the artist s body, which in effect became the medium, as in the performances of Vito Acconci and Bruce Nauman. Artists grouped under this category are often also associated with Land art, Performance, Process art, and other forms of expression that resist the authority of the singular art object.

Definitions from the Guggenheim Museum, New York, New  York