...indefinable. The prolific attempts to define it probably tell us more about its nature than any one definition. Nevertheless, there are some great lines trying to fill in this preeminent blank. Here’s a goodly number of them. The list doesn't include Dante Gabriel Rossetti’s line from the first in the House of Life sonnet sequence: “A sonnet is a moment's monument.” That to me is as good a definition as any.
Indefinability, though, is a palpable quality, not the absence of a quality. Saying we don't know the nature of something is not the same as saying it has no nature. American poet Marianne Moore, admired by TS Eliot, said “I see no reason for calling my work poetry except that there is no other category in which to put it.” Poetry shouldn't be a basket into which anything indefinable can go. The same applies to art. Art is not that which falls short of reality but that which goes beyond it.