An impactor is a spacecraft designed to impact a celestial body. Sometimes the plume produced by the impact is studied by ground-based equipment and/or other spacecraft. Impactors can help characterize the surface of a celestial body or access the subsurface for scientific investigation. Impactors can also theoretically be used to deflect asteroids from their current trajectory into a preferred trajectory. (Bhaskaran and Kennedy 2014)
Bhaskaran, Shyam, and Brian Kennedy. "Closed loop terminal guidance navigation for a kinetic impactor spacecraft." Acta Astronautica 103 (2014): 322-332.