What is a line of poetry commonly referred to as?

A line of poetry is commonly referred to as a verse. This term denotes not just a single line, but is often used in the context of a composition that is written in a rhythmical and metrical form. The use of "verse" highlights the structured nature of poetry, distinguishing it from prose, which typically does not adhere to such rhythmic constraints.

In the context of poetry, a stanza refers to a grouping of lines often sharing a common theme, while a couplet specifically denotes a pair of lines that usually rhyme and have the same meter. A ballad, on the other hand, is a longer narrative poem that often tells a story, typically involving themes of love or adventure. Therefore, while all four terms relate to poetry, "verse" specifically points to an individual line, making it the most accurate answer to the question.