1. The continuation of a syntactic unit from one line or couplet of a poem to the next with no pause.
2. the breaking of a phrase, clause or sentence by the end of a line or between two verses.
Enjambment is in contrast with end-stopping,
where each linguistic unit corresponds with the line length.
Meaning flows from line to line, and the reader's eye is pulled forward. Enjambment creates a feeling of acceleration, as the reader is forced to continue reading after the line has ended. However it can also cause some confusion and unnecessary mental pauses within a thought or idea when used in an unusual manner or placement.
T.S. Eliot's poem "Gerontion" is heavily enjambed:
"After such knowledge, what forgiveness? Think now
History has many cunning passages, contrived corridors
And issues, deceives with whispering ambitions"
while Alexander Pope's "An Essay on Criticism", are completely end stopped:
"Nature to all things fix'd the Limits fit,
And wisely curb'd proud Man's pretending Wit:"
Enjambment is another of the poetic devices that can serve good or evil, depending on the writer's usage, and intentions.
When writing about inner-city construction and traffic, creative enjambment may be just the trick, to add that jumpy jarring effect. When writing about a waterfall, one can create the movement and "flow" with enjambment used in a traditional manner. When writing about the serenity of early morning sunrise, end-stopping is probably more effective.