A line is a unit of writing into which a poem or play is divided: literally, a single row of text. The use of a line operates on principles which are distinct from and not necessarily coincident with grammatical structures, such as the sentence or single clauses in sentences. Although the word for a single poetic line is verse, that term now tends to be used to signify poetic form more generally. A line break is the termination of the line of a poem and the beginning of a new line.

The process of arranging words using lines and line breaks is known as lineation, and is one of the defining features of poetry. A distinct numbered group of lines in verse is normally called a stanza. A title, in certain poems, is considered a line.

Conventions that determine what might constitute line in poetry depend upon different constraints, aural characteristics or scripting conventions for any given language. On the whole, where relevant, a line is generally determined either by units of rhythm or repeating aural patterns in recitation that can also be marked by other features such as rhyme or alliteration, or by patterns of syllable-count.

In Western literary traditions, use of line is arguably the principal feature which distinguishes poetry from prose. Even in poems where formal metre or rhyme is weakly observed or absent, the convention of line continues on the whole to be observed, at least in written representations, although there are exceptions (see Degrees of license). In such writing, simple visual appearance on a page (or any other written layout) remains sufficient to determine poetic line, and this sometimes leads to the suggestion that the work in question is no longer a poem but "chopped up prose". A dropped line is a line broken into two parts, with the second indented to remain visually sequential.

In the standard conventions of Western literature, the line break is usually but not always at the left margin. Line breaks may occur mid-clause, creating enjambment, a term that literally means 'to straddle'. Enjambment "tend[s] to increase the pace of the poem", whereas end-stopped lines, which are lines that break on caesuras (thought-pauses often represented by ellipsis), emphasize these silences and slow the poem down.

Line breaks may also serve to signal a change of movement or to suppress or highlight certain internal features of the poem, such as a rhyme or slant rhyme. Line breaks can be a source of dynamism, providing a method by which poetic forms imbue their contents with intensities and corollary meanings that would not have been possible to the same degree in other forms of text.

Distinct forms of line, as defined in various verse traditions, are usually categorised according to different rhythmical, aural or visual patterns and metrical length appropriate to the language in question. (See Metre.)

One visual convention that is optionally used to convey a traditional use of line in printed settings is capitalisation of the first letter of the first word of each line regardless of other punctuation in the sentence, but it is not necessary to adhere to this. Other formally patterning elements, such as end-rhyme, may also strongly indicate how lines occur in verse.