A reel is one of the vertical columns of symbols on a slot. On mechanical machines reels were physical drums; on online and video slots they are animations that display a result already chosen by the RNG.
The number of reels and rows defines the visible grid. Classic games use three reels, most video slots use five, and many newer formats use six or more, sometimes with each reel showing a different number of symbols, as in Megaways.
Each reel draws its symbols from a reel strip, the full sequence of symbols on that reel. Different reels in the same game often have different strips, which is one way designers control how often high-value combinations land.
Reels usually stop left to right, and in most games wins are counted from the leftmost reel. That order is presentation: the outcome does not change while the reels are spinning, and on GB-licensed online slots features that let you stop reels early are not allowed.