Stack
The money a player has on the table right now. Only that money is in play: you cannot add to it mid-hand, and it caps every bet, yours and his.
The stack is everything you can play with in this hand. At NL100 you sit down with $100, a hundred big blinds, and that number matters more than the stake: with a hundred blinds an all-in before the flop is rare, with twenty it is routine. You cannot lose more than your stack in any run-out, and that is the whole point of a stake.
Stacks are counted in blinds, not dollars, because stack depth decides how many streets can be played. With a hundred blinds a hand has a flop, a turn and a river. With twenty, after one raise and one bet the money is gone, and the flop decision becomes the last one. A shorter opponent's stack caps your bets too: you cannot win more from him than he has.
An example from session 10
9♣8♣ on a board of 2♠ 6♣ Q♥ 9♠; the opponent bets $14.03 and I answer with an all-in of $80.25. He has $14.22 behind: my bet is five times what he can actually pay. By the time he decides the pot is $84.78 and he needs to add $14.22, so being right 14% of the time is enough. He paid with A♦A♠ and was right. The size of the bet was set by his stack, not by my hand.
The same in business
The stack is the resource you have now, not the one you were promised. A round not yet closed, a credit line not yet confirmed are not in the hand. Planning on money that is not on the table is playing a hundred-blind hand with twenty: the streets run out before the plan does.
The whole hand is in session 10: “Caution takes fewer steps, and each one costs more”.
The other terms are in the glossary.