What is Contraction ?
Contraction is (noun) 1. an act of becoming shorter or smaller Light will provoke the contraction of the pupil of the eye. Cold will cause the contraction of metal rails. 2. the movement of the muscles of the womb, when a baby is being born The first strong contractions will come about every twenty minutes. She had two contractions in the car on the way to the maternity hospital.
source: Easier English, Student Dictionary Upper Intermediate Level