Definition of bet
Bet literally means "bet". There are several types of bets in poker (see second paragraph). A bet is a voluntary bet that a player makes. It should not be confused with limping preflop, which is the act of paying the blind. In poker, "betting" is the act of putting up money after the flop. For example, if the flop comes down and the pot is £10, I might make a £5 bet. Any bet in poker is called a "bet".
The different types of bets
If you bet in a pot and are the first to do so, it is a bet. If your opponent raises you, it is theoretically a 2bet, although the term is almost never used. In more common terms, it's called a raise. And if you really like your hand and raise your opponent's raise, it's called a 3bet. The next bet is, you guessed it, the 4bet. In 99% of the cases, the 5bet will be an all in even though theoretically, with very deep stacks, there is no limit to the number of bets you can make.