The pattern of stressed and unstressed syllables in a line of verse or (less often) prose. Regular rhythm is called meter.

In some languages, such as classical Greek and Latin, rhythm is based on the length or even pitch of the syllables. In English, although there have been experiments with "quantititive verse" (where the amount of time it takes to speak each syllable determines "long" or "short"), especially in the Renaissance, the more common basis of rhythm is stress. Stressed syllables in a line can marked with an acute accent ('), which real scansion nerds call an ictus, to show the rhythm, as here:

Róund abóut the cáuldron gó,
Ín the póisoned éntrails thrów.
Even though, properly speaking, English meter doesn't base its meter on "long" and "short" syllables (even though stressed syllables tend to be longer), the terms "short" and "long" are sometimes used. They're mostly holdovers from classical (Greek and Roman) metrical schemes.

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.