Loot table
A weighted list of what a defeated monster or opened chest can drop, and how often.
Each row is an item, or a pointer to another table, with a weight attached. The engine sums the weights, rolls a number in that range and walks down the list until the running total passes it. Because rows can point at sub-tables — "drop something from the rare weapons table" — a designer can rebalance an entire economy in one place instead of touching every enemy. Diablo (1996) turned randomised items with randomised affixes into a genre, and its descendants have been doing the same arithmetic ever since.
Pure chance feels unfair in practice: at a one percent drop rate, one player in three hundred will go three hundred attempts with nothing to show for it. Hence the "pity timer", which nudges the odds up after every miss. Sell the same mechanic for real money and it becomes a loot box, at which point it is a matter for gambling regulators rather than game designers.