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.