Usually the line in the middle of the slot window but also it can be three lines, five lines or even more on video slots. Only symbols on a pay line will result in a win.

an invisible line that moves across the reels of a slot. Winning combinations only pay out prizes when they appear on one of these invisible lines. Must be activated by the player, usually with a wage [..]

The line at which the symbols must align to determine a win or not.

The set combinations of symbols that result in a payout for the player. Most machines and online slots have multiple pay lines

The line or tunnel section which constitues limits of payment for excavation and concrete lining. Overbreak is any excavation beyond the pay line, and overrun is any excess concrete placed beyond the pay line.

The line, or tunnel section, which constitutes limits of payment for excavation and concrete lining. Overbreak is any excavation beyond the pay line, and overrun is any excess concrete placed beyond the pay line. May also be referred to as the "B" line.

the line on which symbols must align in order to achieve winning combinations

The line of numbers and symbols after the reels are spun. The player places a bet on one or more pay lines before spinning.