A verse form, rhyming aa bb cc dd. Rhyming couplets in iambic pentameter are sometimes called heroic couplets.

A couplet meant to stand alone as an entire poem is sometimes called a distich (meaning "two lines"), as this one:

There was a young man from Hong Kong,
Who thought limericks were too long.

From the Guide to Literary Terms by Jack Lynch.
Please send comments to Jack Lynch.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.