Outside bet is a casino term used by roulette players, and it represents various types of bets you can make during a game of roulette.
If you have played roulette before, you might have noticed that the layout of the roulette table is, in a way, split into two areas. One of those areas is known as the inside betting area, while the other holds outside bets.
The layout of the markings printed on the cloth of the roulette table can change from one manufacturer to another. However, if you were to bet on the first, second, and third 12 numbers, 1 to 18, 19 to 36, or made an even/odd or a red/black type of bet, you would be making an outside bet.