Formalism is the doctrine in artistic and literary criticism, but also in philosophy more generally, that holds form in ascendancy, subordinating content, context, or, in a specific sense, meaning -- meaning in the sense of standing in for a real world concept or sentiment. (In linguistic terminology, I would say syntax over semantics, but I'm not sure how well the correspondence holds). Though I, personally, could not disagree more with this doctrine (in literature anyway), even some proponents of form over substance think Formalism as a doctrine goes too far: Peter Viereck most eloquently wrote in his essay "Form in Poetry" that "Formalism, by being an -ism kills form by hugging it to death, whereas formlessness kills it openly".
I find the history of Formalism, by that name, in art and literature to be anecdotal rather than narrative. In the visual arts, its practice is epitomized by the abstractionists. In poetry, there seems to be a consistent pendulum swing of Formalist surges and backlashes and the term is applied loosely and pejoratively. But regardless of labels, the relative ascendancy of form over substance ebbs and flows in the practice and criticism of art, and it is a major axis upon which to differentiate artistic movements.