Definition of a 3bet
A 3b or 3bet is the third bet one makes at a poker table on a given street. The mandatory blinds to be placed preflop determine the first bet. Calling or "limping" will be referred to as the first bet (or 1 bet, although this term is almost never used). If a player raises to say 3 times the size of the big blind, it is a 2 bet, commonly called a raise. If another player over-raises the raise to say 10 big binds, it is called a 3bet. The 3bet can take place preflop, on the flop, on the turn or on the river.
A concrete example of a 3bet
We are at a 6-player table. This is a 1 player cash game. The player in UTG position (the first player to talk preflop) opens to 5 (he raises rather than just paying the blind of 2 so he 2bets). Some players fold to the button who raises the raise (3bet) to 20. The player on the button has made a 3bet. The same thing can happen on any street (flop or turn or river). If a player bets 5 on the flop in a pot of 10, his opponent raises to 30, and the player who originally bet 5 raises to 70, he would be said to have 3bet the flop.