Radiance are the smaller components of the Radiance Crystals that scattered all around the Radiant Isles in the 3DS version of Skylanders: Spyro's Adventure. Collecting them are one of the five objectives in each level. There are a total of 135 Radiance in the game.
It appears in Skylanders: Battlegrounds as Energy. You get it by harvesting crops. It is used by your Skylanders to complete tasks.