Free verse
Verse freed from regular meter and mandatory rhyme — but not from rhythm: the line break becomes the central tool.
Free verse frees itself from regular syllable count, mandatory rhyme and fixed forms. Born in France at the nineteenth century's end, carried in English by Whitman then the modernists, it became contemporary poetry's dominant regime.
Free doesn't mean unconstrained: deprived of meter, free verse rests entirely on local decisions — where to break the line (each break is a micro-event: see enjambment), which sound returns (assonance, anaphora), what stanza breathing. It's a responsibility more than an exemption: in regular verse the form decides; in free verse every line must justify its break. The merciless test: copied out as prose, does the poem lose something? If not, it was prose.
Example
Whitman's long unrhymed lines in Leaves of Grass: breath itself as the measure.