Megaways
Megaways is a reel mechanic originally developed by Big Time Gaming and licensed to dozens of other providers. The defining feature: the number of symbols on each reel changes randomly every spin. One reel might show 2 symbols, the next might show 7, and the total number of ways to win shifts constantly. The maximum is usually 117,649.
This mechanic creates a very different feel compared to fixed-payline slots. Some spins give you 64 ways to win, others give you 100,000+. Big wins tend to happen when most reels land near their maximum height, which is rare. That's why most Megaways slots are high variance.
Bonanza was the first Megaways slot and it's still one of the best. Since then, providers like Pragmatic Play (Gates of Olympus), Red Tiger (Gonzo's Quest Megaways), and Blueprint Gaming have built their own versions. Some of these are excellent. Some are lazy reskins. Check the reviews before you commit your balance.