A measure of thermal energy in units of degrees or electron volts. 1 electronvolt is about 10,000 Kelvin. At an atomic level temperature equates to movement of the atoms – hotter plasma has faster-moving ions and electrons in it. High temperatures are important for fusion because fast movement of ions means that during a collision the ions get close enough to fuse.
Temperature is one of the three parameters that make up the triple product, along with number density and confinement time.