Training Terminology: Compound vs. Isolation

Exercises are described as being "compound" or "isolation" exercises. But what's the difference?

It's a matter of joints.

Compound exercises involve movement around more than one joint. For example, a back squat involves movement around the ankle joint, the knees, and the hips.

Isolation exercises involve movement around a single joint. The classic biceps curl only involves flexion of the elbow, thus isolating the single joint.

That's the whole basis of the split - the number of joints involved. You'll occasionally see "isolation" exercises defined as "single-muscle exercises" and "compound" as "multiple muscle exercises" but this isn't a proper definition. Isolation exercises do not truly isolate one muscle, but rather one joint (and thus emphasize a very small number of muscles). Compound exercises are not defined as compound based on the number of muscles involved, just the joints.

If you need to determine if a given exercise is compound or isolation - and it's not immediately obvious - check the directory of exercises at EXRX. The site helpfully labels all exercises as compound or isolation.