recoil

Short definition from Wiktionary

Noun

1. 1
   A starting or falling back; a rebound; a shrinking.
2. 2
   The state or condition of having recoiled.
3. 3
   The energy transmitted back to the shooter from a firearm which has fired. Recoil is a function of the weight of the weapon, the weight of the projectile, and the speed at which it leaves the muzzle.
4. 4
   An escapement in which, after each beat, the scape-wheel recoils slightly.

Verb

1. 1
   To pull back, especially in disgust, horror or astonishment.
2. 2
   To recoil before an opponent.
3. 3
   Of a firearm: to quickly push back when fired.
4. 4
   To retire, withdraw.