How we determine what is a poem is a matter of cultural consensus. While we can determine whether something is a cat or a supernova by using various absolute scientific standards, poetry is not like biology or physics in that it depends on culturally variable types of definition.
Ezra Pound's poem "In the Station of the Metro" was initially published in 1913 in Poetry, a literary journal. It was intended to exemplify the then-radical "Imagist" literary movement. In his manifestos and other critical works, Pound argued the heart of poetry was the "image," "that which presents an intellectual and emotional complex in an instant of time." According to this definition, "In the Station of the Metro" is an imagist poem, distilling the experience of the subway, and particularly of people standing and hanging on to straps in the subway, through a single metaphor of damp, dying petals.
This work fits the definition of poetry in two ways. First, it is widely reprinted and taught as a poem, making it part of our cultural norms of what constitutes an example of poetry. Secondly, it fits one important critical definition of a poem.