A kind of metrical foot. A dactyl is a stressed syllable followed by two unstressed syllables: for example, "Canada," "holiday," "camouflage." (The name comes from the Greek for "finger" — as in pterodactyl, "winged-finger" — and you can remember the pattern by thinking of the three joints in a finger: long, short, short.)
Because a dactyl has three syllables, it's called a triple meter.
Classical epic poems were traditionally written in both Greek and Latin in dactyllic hexameter.