A supernova remnant is the debris ejected from a supernova, essentially forming a type of nebula. It consists of material from the exploding star ejected, often at relativistic speeds. It may be optically visible if a source of light is nearby that is reflected, or if it collides with other material, causing one or both to heat from the shock wave.
A different term, stellar remnant, is used for a black hole or neutron star left from a supernova. These may be observed long after the supernova remnant has dissipated.