A line break in poetry is the termination of the line of a poem, and the beginning of a new line.
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.
An example may be taken from E.E. Cummings' poem 'old age sticks'
scolds Forbid
den Stop
Must
n't Don't
The line break within 'must/n't' allows a double reading of the word as both 'must' and 'mustn't', whereby the reader is made aware that old age both enjoins and forbids the activities of youth. At the same time, the line break subverts 'mustn't': the forbidding of a certain activity—in the poem's context, the moral control the old try to enforce upon the young—only serves to make that activity more enticing.
While Cummings's line breaks are used in a poetic form that is intended to be appreciated through a visual, printed medium, line breaks are also present in poems predating the advent of printing.
Examples are to be found, for instance, in Shakespeare's sonnets; however, some Early Modernists would argue that such an effect wasn't consciously intended by Shakespeare to be read as line breaks, which arise from the advent of printing as a method of distribution, which has a contextual effect upon that which is to be distributed. Here are two examples of this technique operating in different ways in Shakespeare's Cymbeline:
In the first example, the line break between the last two lines cuts them apart, emphasizing the cutting off of the head: