ETYM Latin fissilis, from fissus, p. p. of findere to split. Related to Fissure.
Capable of being split or cleft or divided in the direction of the grain.

Množina: fissile materials

A material consisting of atoms whose nuclei can be split when irradiated with low energy (ideally, zero energy) neutrons. Well-known examples are plutonium-239 and uranium-235.