Recoil is the 'kick' felt by a firearm when it is fired. It is the rearward motion of the firearm produced by its discharge. The severity depends on calibre, weight and velocity of the round. Other factors are weight of the rifle (the lighter the rifle the more recoil), the design of the stock and use of a suppressor.
In technical terms, recoil is the backward momentum equal to the forward momentum, by the law of conservation of momentum and or Newton's Third Law of Motion (M = M). The recoil is caused by the gun's backward momentum, which exactly balances the forward momentum of the ejecta of the gun (the projectile/s, wad, sabot, propellant gases, and so on). In most small arms, the momentum is transferred to the ground through the body of the shooter; while in large arms such as mounted machine guns or cannons, the momentum is transferred to the ground through a mounting system.
It is common to calculate a firearms momentum and thus the recoil. However, recoil as a calculation is seldom that of momentum (M = md/t ). Rather, the calculation is [translation] kinetic energy from Newton's Second Law of Motion (F = ma). In simple terms, recoil is a calculation or more correctly a measurement of the energy felt when a firearm kicks. This measurement is called “recoil energy” or free recoil.