From Wikipedia

Walt Whitman

Walter "Walt" Whitman
 (May 31, 1819 – March 26, 1892)
 was an American poet, essayist and journalist. A humanist, he was a part of the transition between transcendentalism and realism, incorporating both views in his works. Whitman is among the most influential poets in the American canon, often called the father of free verse. His work was very controversial in its time, particularly his poetry collection Leaves of Grass, which was described as obscene for its overt sexuality.

Whitman's work breaks the boundaries of poetic form and is generally prose-like. He also used unusual images and symbols in his poetry, including rotting leaves, tufts of straw, and debris. He also openly wrote about death and sexuality, including prostitution. He is often labeled as the father of free verse, though he did not invent it.