Core loop
Also said as Gameplay loop, Compulsion loop
The short cycle of actions a player repeats the most, where finishing one round sets up the next. Fight, get loot, buy gear, fight something bigger.
This is most of what a player does for most of their hours. If the loop is not good by the tenth lap, no amount of content will rescue it.
Say it
“Core loop is dig, sell, upgrade, dig deeper, about ninety seconds a lap. Selling must always pay for something that makes the next lap better.”
Where you’ve seen it
Jaime Griesemer of Bungie described Halo as "30 seconds of fun" repeated over and over in different places, with different weapons and enemies. Monster Hunter runs on a longer one: hunt, carve, craft, then hunt something bigger.
Not to be confused with game loop
The core loop is what the player repeats, over seconds or minutes. The game loop is the code that runs once a frame.