Commonly used to refer to an initial reraise before the flop. The term has its origins in fixed-limit games where an initial raise is worth two bets, then the reraise is equal to three and so on. Similarly in no-limit games the big blind is the first (forced) bet, the first raise is the second, and the first reraise a “three-bet.”
After that, the next reraise is called a “four-bet” and so on. Thus when someone is described as “five-bet shoving all in,” it is understood that the all-in raise came following a series of raises back and forth.