A lepton is a distance measuring unit in Tiberian Sun and Red Alert 2, which equals 1/256th of a map cell's edge. So, a single map cell is 256x256 leptons. Leptons abide by the pseudo-isometric perspective of the game, i.e. the leptons-per-pixel ratio is greater vertically than horizontally.
This unit is used to measure object Firing Offsets, animation draw offsets, and such distances which require a high level of sub-cell precision.