A stanza is a grouping of lines that forms the main unit in a poem.
From A Poet’s Glossary
The following definition of the term stanza is reprinted from A Poet's Glossary by Edward Hirsch.
The natural unit of the lyric: a group or sequence of lines arranged in a pattern. A stanzaic pattern is traditionally defined by the meter and rhyme scheme, considered repeatable throughout a work. A stanzaic poem uses white space to create temporal and visual pauses. The word stanza means “room” in Italian— “a station,” “a stopping place”—and each stanza in a poem is like a room in a house, a lyric dwelling place. “The Italian etymology,” Ernst Häublein points out in his study of the stanza, “implies that stanzas are subordinate units within the more comprehensive unity of the whole poem.” Each stanza has an identity, a structural place in the whole. As the line is a single unit of meaning, so the stanza comprises a larger rhythmic and thematic sequence. It is a basic division comparable to the paragraph in prose, but more discontinuous, more insistent as a separate melodic and rhetorical unit. In written poems stanzas are separated by white space, and this division on the printed page gives the poem a particular visual reality. The reader has to cross a space to get from one stanza to another. Stave is another name for stanza, which suggests an early association with song.
A stanza that consists of lines of the same length is called an isometric stanza. A stanza that consists of lines of varying length is called a heterometric stanza.
A stanza of uneven length and irregular pattern—of fluid form—is sometimes called quasi-stanzaic or a verse paragraph.
The monostich is a stanza—a whole poem—consisting of just one line. After that, there is the couplet (two-line stanza), tercet (three-line stanza), quatrain (four-line), quintet (five-line), sestet (six-line), septet (seven-line), and octave (eight-line). There are stanzas named after individual poets, such as the Spenserian stanza (the nine-line pattern Spenser invented for The Faerie Queene, 1590–1596) and the Omar Khyyám quatrain (the four-line stanza the Persian poet employed in the eleventh century for The Rubáiyát). Each stanza has its own distinctive features, its own music, and its own internal history that informs and haunts later usage.