You probably came to this page by accident, because everyone knows what a hole is. But there are some people who don't know their ass from a hole in the ground, and they have spent an inordinate amount of time contemplating what a hole really is (an activity which is known as navelgazing).
Holes in topology
Holes in sexology
In sexology, a hole is something a man could conceivably fuck. The vagina is the ideal hole, and the standard by which all other holes are judged. Under this definition, a drinking straw has two absolutely terrible holes for a man with a very small dick. Women have like four or five fantastic holes and several more great ones. Men have zero holes, unless you're gay or in prison.
There is a branch of research combining both topology and sexology (something previously thought impossible). This field, which defines a hole as "something you can fuck all the way through", is known as hentai.