If an isotope(such as uranium 235) can support fission, one might say it is fissile. What would you call a material like deuterium can support fusion, what would you call it?
U-235 and Pu-239 are fissile, or fissionable, as are other isotopes (e.g., U-233, Pu-241). D and T are fusionable. I don’t think fusionable is used very much, but see Patents, Fusionable Material Target, http://www.google.com/patents/US20060198487. “Can support fission” or “can support fusion” is a strange wording. Better to say “can undergo nuclear fission” or “can undergo nuclear fusion.” I could go on, but I won’t.
Upon reflection, “can support nuclear fission” is OK for U-235 and Pu-239. Wilipedia defines fissile as “In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of any energy.”
Of course, a lot of engineering goes into the sustaining or supporting part, but it has been achieved. As for fusion, we don’t have fusion reactors yet.