A kilonova (or macronova) is an astronomical flash of EMR corresponding to 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 EMR 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.