The words "modern" and "modernist" are both disputed descriptions of poetry and several other arts. Matters for dispute include the period covered and the characteristics of modernity and modernism. G. K. Chesterton, who wrote traditional poetry at the time when such modernists as T. S. Eliot and Ezra Pound were conducting their experiments, famously complained that "modern" was an absurdly weak and unphilosophical description to attach to a way of writing. In "The Case for the Ephemeral," he wrote:
It is incomprehensible to me that any thinker can calmly call himself a modernist; he might as well call himself a Thursdayite.
Chesterton's own verse, however, makes the opposite point. He is not a modernist, even though his poems were written during the modernist period. There must, therefore, be some definite characteristics which identify modernism as a school and a style, and these can be found by comparing Chesterton and other traditional poets to Eliot and the modernists.
First, the modernists tended to use free verse. This is not invariable, and they certainly did not dispense with rhyme, meter, and form altogether. However, they were willing to vary the form to suit the subject, as Eliot does in "The Love Song of J. Alfred Prufrock." Second, there is a complexity and doubtfulness about modern poetry which is not found in more traditional verse. Take "The Love Song of J. Alfred Prufrock" again. This is not a simple poem about unrequited love, like an Elizabethan lyric in which the poet assumes that all would be well if his mistress returned his feelings. Prufrock questions everything and can see no possibility even of revealing his love—an act which seems to him to be a cataclysm which would "disturb the universe."
Beyond these two characteristics, modern poetry was reflective of the age in which it was written in a way that no previous school or period had been. One can argue that Kipling and Tennyson incidentally tell the reader quite a lot about what it means to be a Victorian, but this is not their subject, precisely because their Victorian values remain unquestioned. The equivocal, doubtful tone characteristic of modern poetry means that the reflection of the age is usually an uncomfortable one, as when Eliot, in "The Waste Land," continually contrasts the heroism and splendor of past ages with the squalor of London in the twentieth century.