A Kilonova (or Macronova) is an explosion more energetic than an ordinary Nova but less than a Supernova. It stems from the collision of two dense bodies. Typically it is 1000 times brighter than a nova but a tenth or a hundredth of a supernova.
Much of the Electromagnetic Radiation from a kilonova is thought to occur in the Infrared, making OIR observation useful.
GW170817 is classified as a kilonova, confirming the theory that Neutron Star mergers produce them. The Transient faded more rapidly than a supernova.