means The line across a slot machine's reels along which matching symbols must land for a player to win a payout.

from A plain compound of 'pay' and 'line,' born from the mechanical slot machines of the late 19th and early 20th centuries — those one-armed bandits where a single horizontal line across the spinning reels decided your fate. 'Pay' traces back through Old French 'paier' to Latin 'pacare,' meaning to pacify or satisfy (settle a debt and you settle the peace); 'line' comes from Latin 'linea,' a string of flax. As machines grew more elaborate, one payline became many, but the word held its literal sense: the line on which payment is decided.