To supplement the Webster definition, more specifically:
In Latin poetry, the practice of omitting unnecessary vowels to fit the meter when the poetry is being read aloud. This occurs when one word ends with a vowel (or a vowel followed by an 'm', which to a Roman was really just another, nasal vowel), and the next one begins with a vowel (or an h followed by a vowel, since the Romans didn't pronounce their h's very strongly), and so the end of the first word is elided (dropped). Note: this doesn't usually happen across a caesura, and never between the end of one line and the beginning of another. Also, elision doesn't always have to occur, and the deliberate avoidance of it is called hiatus.
Examples:
- in vento et rapida => in ventet rapida
- Nunc iam illa non volt => Nunc iilla non volt
- ibi illa=> ibilla