What is the overhead press?

In one line

The overhead press is a shoulder exercise where you push a barbell or dumbbells from shoulder height up over your head until your arms are straight.

What is the overhead press?
The exercise where you press a barbell over your head
At the gym you hear names like 'overhead press', 'military press' and 'shoulder press'. This post explains what the exercise is, which muscles it uses, and what changes between versions.
In a nutshell
Press a weight from shoulder height to overhead
The overhead press is an exercise where you push a barbell or dumbbells held at shoulder height up over your head until your arms are straight. It is done standing or seated. It is also called the 'military press' or 'shoulder press'.
What kind of movement is it?
The shoulders, elbows and shoulder blades move together
As you press the weight up, the arms lift upward at the shoulders and the elbows straighten. The shoulder blades, the flat bones on the upper back, also rotate upward. Because several joints move together, the overhead press is a compound exercise.
The main muscle: the deltoid
The muscle that wraps around the shoulder
The deltoid is the triangle-shaped muscle that gives the shoulder its rounded shape. Its job is to lift the arm out to the side and upward. It is described in three parts (front, side and rear), and the overhead press uses all three.
Muscles that help
The triceps and the upper trapezius
The triceps on the back of the upper arm, which straighten the elbow, help push the weight up. The upper part of the trapezius, the muscle covering the back of the neck and upper back, also works; this is the part that shrugs the shoulders. The chest muscle is also listed among the working muscles.
Standing vs seated
Standing, the abs help hold you steady
Standing, the trunk has to hold itself steady without a backrest. In a study with a heavy dumbbell in each hand, the front abdominal muscle worked about five times more standing than seated. The standing version also uses the abdominal muscles.
The bar's path
In front of the face, or behind the neck
The barbell can be pressed so it passes in front of the face or behind the neck. Pressing in front of the face made the chest muscle work more. Pressing behind the neck made the rear deltoid work more.
A common misconception
Does the version where you lift the most use the shoulders most?
No. In a study comparing standing vs seated and barbell vs dumbbells, standing with dumbbells was the version where the heaviest weight lifted was the lightest. Yet overall, the deltoids worked the most in this version. The researchers explained that it is the hardest version in which to keep the weight from wobbling.
Summary
A shoulder exercise of pressing overhead
The overhead press is pressing a weight from shoulder height to overhead until the arms are straight; the deltoid is the main muscle, with the triceps and upper trapezius helping. In a comparison study, the version where the shoulders worked most was the one with the lightest weight. Related terms: compound exercise, bench press, range of motion.