In talk about atoms, fissile means this kind of material can be split in nuclear fission — the splitting of the centre of an atom. Uranium-235 and plutonium-239 are the famous fissile stuff used in power stations and, sadly, in bombs. Though actually, careful nuclear scientists draw a thin line: fissile often means it can split after catching a slow neutron; fissionable is a wider word that also covers things that need a fast neutron. In everyday English, people blur the two. There is an older, quieter sense in geology: a fissile rock, like shale, splits easily into thin sheets. Same idea — able to be split — different world. The word is a science adjective, not a thing you hold.