After speaking with some of you this afternoon, I have found a few (hopefully helpful) definitions.
Modernism The adjective 'modern' distinguishes ideas and usages of the present and of the recent past from earlier ones, even when these are still generally adhered to. [...] Modernism and Modernist were words attache dto developments in the arts that were opposed to or to some degree divergent from, the ideas and canons represented by the classical tradition, prioritized in the academies. When Modernism in the visual arts began is a matter of dispute. Some make 1863 the key date, with Manet's challenging translation of time-honored mythological subjects into scenes whose meaning is modern and secular; or 1874, with the first Impressionist exhibition, or around 1888, with Gauguin and Bernard's anti-naturalistic style, rejecting the idealism or academic art and its technical refinement in favor of a primitivistic idiom partly derived from folk art, primitivism undermined the principle of imitation of appearances to which sophisticated art had generally aspired. These starting points are all in the history of French art, and there is no denying the role of Paris both as the arena in which French and other artists worked to test and establish new traditions and where a context of critical debate developed around them. Yet one is tempted to associate the origins of Modernism with Romanticism and its challenge, throughout Europe, so the unique authority of classicism and its emphasis on the individual artist as the source or meaning and authenticity. From it came a self-consciousness, in matters of style and other priorities, and slso the urge to form diverging groups or movements new to the history of art yet characteristic of the late 19th and 20th century art. Thie urge was, however, always countered by a contrary urge to individualism in through and practice, all groupings were temporary, the more short-lived the firmer the orthodoxy they appeared to represent even if their names continued to serve as rallying calls.
With the approach of 1900 the awareness of a new beginning in art was heightened by a wider sense of a new age opening to which changes in daily life and expectation of further changes (e.g. electricity for light and power, radio, aeroplanes, and agreed global time system, etc. )gave experiential reality. At the same time political upheaval--socialism and anarchism - engaged with systems already weakened by the 19th century's succession of revolutions and wars. Modernism was widely, but not exclusively , associated with life in an industrialized society, and battle was joined with those who preferred art to reflect past ages and their relative stability. By the beginning of the First World War, in 1914, all the cultural centers of Europe and New York were kept aware of dramatic new developments in the arts, and particularly in the visual arts by exhibitions, specialist journals and the press in genera, although the response was largely negative and patronage was remarkably slow to engage. By 1914, it was also clear that Modernism was not one direction in the broad stream of modern art--though it was sometime associated with an overcoming of impressionism as the last style rooted in imitating natural phenomena--but many exhibiting marked differences in theory and practice. [...] Modernism was never one movement, and became more divided as abstraction became prominent.
Around 1970, Post Modernism emerged as a term implying the end of Modernism and anew pluralism in the arts, first in architecture where the so-called International Style had become dominant and was being devalued by replication. Springing from the ideas and methods of major architects such as Le Corbusier, Mies van der Rohe and Gropius, themselves active internationally and of wide influence, the International Style, crudely associated with the notion of functionalism in response to the industrialized world which it served was felt to be stifling regional traditions, alternative building methods and individual hopes of fame through personal innovation, no such central orthodoxy existed in Modernist art, so that the champions of Post-Modernism have needed to demonize Modernism as a stultifying force, solemn and homogenous. Modernism has been accused quite specifically of formalism, of elevating consideration of means and methods over communication.