An exercise that moves one joint and loads mainly one muscle, such as a biceps curl, a leg extension or a lateral raise.
The opposite of a compound lift. Isolation work will never be the base of a program, because it moves little weight and trains little at a time, but it does two things compounds cannot: it adds volume to a specific muscle without adding systemic fatigue, and it trains muscles that compounds under-load, such as the rear delts, the hamstrings in a squat-heavy program, and the external rotators.
A reasonable ratio for a general client is two thirds compound, one third isolation. For a bodybuilding client it moves closer to half, because the goal is the individual muscle rather than the lift.
Higher reps suit it. Sets of 10 to 20 at RPE 8 to 9 on a lateral raise beat sets of 5, where the weight is small enough that momentum does most of the work.