1. the top of a man's penis, or a hermaphodite's penis I guess as well. Also reffered to as a purple helmet.
2. A condom, as worn on the head of your penis, perhaps a more accurate term.
3. A kevlar, or plastic covering for your less important head, as worn while riding a motorcycle or visiting a construction site. Or oddly, enough, while exploring actual caverns. See spelunking
Timmy, what have I told you about playing indoor sports without a helmet? Naughty boy!