1. A shrinking or reducing of something.
2. A tightening or narrowing of a muscle, organ, or other body part.
3. In grammar, a shortened form or shortening of a word or phrase; such as, "he'll" for he will; "they'll" for they will; "won't" from will not; or the phrase, "o'clock" for of the clock resulting in the omission or combining some of the sounds of a longer phrase.

In English, the omitted letter or letters are usually marked with an apostrophe or a period, depending on the type of contraction.