What are compound and isolation exercises?

In one line

A compound exercise is one where several joints move together, and an isolation exercise is one where only one joint moves to work one chosen muscle.

What are compound and isolation exercises?
The 'squats are compound, curls are isolation' you hear about
At the gym or in workout videos you hear phrases like 'compound exercises such as squats' or 'arm isolation exercises'. This post explains what these two words use to divide exercises, which exercises belong to each, and what each one does.
In short
Several joints, or just one joint
A compound exercise is one where several joints move together, and an isolation exercise is one where only one joint moves. In research papers they are called 'multi-joint' and 'single-joint' exercises.
First, joints
Where bones meet and bend or straighten
A joint is a place where bones meet and connect. Shoulders, elbows, hips, and knees are typical joints. When a muscle pulls on a bone, the joint bends or straightens, and the body moves.
Compound exercises
Examples: squat, bench press, leg press
These are exercises where several joints move together, such as squats and bench presses. For example, in a leg press the hip and knee joints straighten together as you push the platform. So several muscles work together in one movement.
Isolation exercises
Examples: biceps curl, leg extension
These exercises move only one joint to pick out one muscle. A biceps curl bends only the elbow, and a leg extension straightens only the knee while you sit. The dumbbell fly, a chest exercise, is also classed as an isolation exercise.
What compound exercises do
Grow several muscles with one movement
In a study where people new to training did leg press with one leg and leg extension with the other, growth of the whole front thigh muscle showed no difference between the two. On top of that, the leg press also grew the buttock muscles and the inner thigh muscles. That is why it is seen as an exercise that grows many muscles in a short time.
A common misconception
Do compound exercises alone grow every muscle?
No. In the same study, one part of the front thigh muscle, the part that also crosses the hip joint, barely grew with the leg press. This muscle grew a lot only in the leg trained with leg extensions. In this study, the isolation exercise filled in the muscle the compound exercise missed.
Summary
Several joints means compound, one joint means isolation
A compound exercise is one where several joints move together, and an isolation exercise is one where only one joint moves. Compound exercises do not grow every muscle evenly. Related terms: training split, training volume, superset.