Impact play is a term used in BDSM for any type of play in which the body is stimulated through an impact (or usually a series of impacts) on the skin in order to create pain. The choice of implement, the body part inflicted, the force, speed and technique all determine the "feel" of the individual impact, and of the play session as a whole. Typically there will be a characteristic balance of sting (surfacial) and thud (deep tissue) elements. Besides the obvious element of pain, other typical physical reactions include reddening, warm glow, and marks.
In practice, the term "impact play" is mostly used to group flagellation and spanking together. Less common forms of impact play include punching and face slapping. Often, a whip or spanking implement is used, but the hand is also an all-time favourite.
In most contexts, "impact play" is synonymous with consensual erotic corporal punishment, because the term corporal punishment is mostly associated with whipping or spanking. However strictly speaking, impact play is only a sub-case of corporal punishment, for corporal punishment also includes non-impact methods to inflict physical pain or discomfort, for example ear pulling, making the subject stand or kneel in uncomfortable poses, mouth soaping, or impactless but pain-inducing BDSM techniques like figging, nipple clamps and wax play.