Inside bet is a term used in both online and brick-and-mortar casinos, and it refers to a type of bet you can place while playing roulette.
If you have ever played roulette, you might have noticed the layout on the roulette table looks like it was comprised of two conjoined parts. One of those layout parts has various bets that appear to be on the inside of the layout, which is how the term inside bets got its name.
Inside bets include betting on specific numbers or small combinations of numbers. Therefore, if you find yourself placing a straight up bet, split bet, street bet, or corner bet, you are making an inside bet.
Naturally, the opposite of inside bets is outside bets, and this type of bets include even money bets such as high or low, even or odd, and black or red, for example.