A supernova remnant is a possible after-effect of a supernova, essentially a type of nebula. It consisting of the supernova's ejected debris (material ejected from the exploding star, often at relativistic speeds), along with interstellar medium affected by the debris. The remnant may be optically visible if a source of light is nearby that is reflected, or if the debris collision with other material causes one or both to heat from the shock.
A different term, stellar remnant, is used for a black hole or neutron star left from a supernova. These remain after any supernova remnant has dissipated.