1:n. the top to a table, most generally taking the shape of a large (ten feet by ten feet, plus or minus seven feet in either or both directions), flat rectangular surface, or circular surface of similar dimension, but sometimes taking whatever shape its maker's whimsy leads to.
2:a. said of a roleplaying game that is neither computerized or live action, and whose rules are encompassed in books (D&D being the canonical example). Once said due to the prevalence of playing them on the sort of tabletop mentioned by meaning 1, this is now more commonly seen as a reference to the average size of these books, and the fact that the books needed to play any one game generally reach to roughly the same height as the average tabletop (meaning 1).