An Endothermic Reaction is a reaction that absorbs heat,
the inverse of an Exothermic Reaction, i.e., that emits heat.
Some nuclear reactions require Kinetic Energy of the involved particles
(i.e., heat), and absorb that heat, turning the energy into
the rest mass of the resulting particles. Such reactions affect
the amount of heat generated Fusion-powered devices (e.g., stars),
and also contribute to collapsing cores, due to loss of pressure
due to absorption of heat, e.g., in Core Collapse Supernovae.