When there is an impact of two bodies during a short duration of time, an equal and opposite force is exerted by one body on the other. In such a context, the product of Force and time duration is called the impulse of the impact or collision.
Impulse = Force * Δt = ( Δp / Δt ) * Δt = Δp
where p = momentum and Force is rate of change of momentum.
Hence, impulse is the change in momentum of the bodies when a collision occurs between them during a very short period of time.
S.I. unit is same as the unit of momentum: kg-m/sec or N-sec