Exercise where the speed of movement is held constant by a machine, so resistance matches whatever force the muscle produces at every joint angle.
It requires a dynamometer, a machine that will not let the limb accelerate past a set speed, for example 60 degrees per second. Push harder and the machine pushes back harder; the speed does not change.
This makes it excellent for testing and largely irrelevant for training. Clinics use it to measure a hamstring-to-quadriceps strength ratio after a knee injury, where a ratio under about 0.6 flags a return-to-play risk. Almost no commercial gym owns one.
The nearest thing a trainer has is a heavy sled push or a swimming pull, where the resistance rises with the effort. Neither is truly isokinetic, but both share the useful property that the client cannot cheat by accelerating.