A warhead is the front part of a missile, torpedo, or rocket — the part that carries the explosive and does the damage. The rest of the missile is only the engine and the guidance that carry this part to the target. People say "nuclear warhead" very often, because when countries count how much power they have, they count warheads, not missiles. One missile can carry several warheads, so the numbers are different. You will also meet "conventional warhead", which means an ordinary explosive one, not nuclear. The word is a normal countable noun: one warhead, ten warheads. It belongs to news and military talk, and it turns up constantly in reading about weapons agreements — "reduce the number of warheads", "deploy warheads", "a warhead was fitted". Outside that subject it is almost never used, though people sometimes joke about a person being a "warhead" — meaning someone who loves a fight — but that is slang and not common.