Definition of stack
A stack is the player's set of chips. If you are in games of cash games, let's say on a table where the blinds are 1/2 euro, your stack will be 200 euro. If you go all in during a hand (or stack hand, one might say) at one stage of the hand, you will play your entire stack. If you lose the hand, you will have to play all restack of you to continue playing. In English, the equivalent expression is a "stack". The word stack, although English, is sometimes used in Quebec and France. The expression stack is used in both money games and tournaments.
Determining the size of the stack
The size of the stack is determined by the size of the blinds. The maximum stack for a game in which the blinds are 1/2 euros will be 200 euros, or 100 blinds. In tournaments, the size of the stack will be determined by the tournament organizers. When you pay your entry fee for the tournament, you will get your stack. If the tournament allows it (tournament with the possibility of restack), in case you lose all your chips, you will be able to restack, i.e. pay your entry fee again to get the initial stack.