What is a nuclear warhead? What is fissile material?

The atomic bomb relies on fission for its power. “Nuclear fission is the splitting of the nuclei of heavy atoms such as uranium or plutonium.”

The thermonuclear bomb relies on fusion.  “Nuclear fusion is the combination of light atoms such as hydrogen isotopes.”

“In both processes part of the mass of these elements is converted into energy and, if this can be made to happen fast enough, a nuclear explosion is the result.”

(Source for the above: Christy Campbell, Nuclear Facts, Methuen, Toronto, 1984.)

A "warhead” is the explosive part of a nuclear weapons system.  "Warheads consist of nuclear materials, conventional high explosives, related firing mechanisms and containment structure” (as defined within the model Nuclear Weapons Convention).

"Fissile Material" are elements in which the nuclei may be split either spontaneously or with the bombardment of neutrons of low energy.

"Fissionable Material" are elements in which the nuclei may be split either spontaneously or with the bombardment of neutrons regardless of the energy of the neutron, and includes fissile material.

See additional definitions in the model nuclear weapons convention document.