Kick
The conventional way of drilling is to maintain an overbalance where the pressure (force per unit area) in the wellbore while drilling is maintained above the formation pressure. The difference between the hydrostatic pressure in the well and the fluid pressure in the formation being drilled is called 'Overbalance'. Overbalance is considered the primary barrier to well control and is maintained to disallow the flow of formation fluid into the wellbore. If for any reason, the overbalance is lost and the hydrostatic pressure drops below the formation pressure, the formation fluid can enter the well. This flow of fluid from the formation into the wellbore is called kick or influx.