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