What is a squat?

In one line

A squat is an exercise in which you bend your knees and hips to sit down from standing and then stand back up, using several leg muscles together.

What is a squat?
The exercise where you sit down and stand up
At the gym you often hear "leg day starts with squats." People squat with just their body weight or with a heavy barbell on their shoulders. This post explains what the squat movement is and which muscles it uses.
In one line
Stand, sit down, and stand back up
A squat is an exercise where you stand with your feet apart, bend your knees and hips to sit down, and then stand back up. People do it with body weight alone, or add weight by resting a barbell on the shoulders behind the neck. The Korean standard dictionary spells the word a little differently from the usual gym spelling.
Several joints at once
Hips, knees and ankles bend together
A joint is where two bones meet and can bend and straighten. When you squat down, the hip joint (where the leg meets the trunk), the knee and the ankle all bend, and they all straighten as you stand up. That is why the squat is classed as a 'compound exercise', one where several joints move together.
Main muscles ①
The front of the thigh: the quadriceps
The muscle that straightens the knee as you stand up is the large muscle on the front of the thigh, the 'quadriceps'. It is made of four muscles working as one. In studies where people squatted for several weeks, this muscle clearly grew.
Main muscles ②
The buttocks and the inner thigh
The large buttock muscle, the 'gluteus maximus', and the inner-thigh muscles, the 'adductors', also work. In studies where people squatted for several weeks, these two grew along with the quadriceps. So one movement uses several large leg muscles at once.
Where the bar sits
Back squat and front squat
With the barbell on the shoulders behind the neck it is called a 'back squat'; with the bar resting on the front of the shoulders it is a 'front squat'. In a study where people who already lifted weights did both, how hard the muscles worked showed no difference.
A competition lift
One of the three powerlifting lifts
The squat is one of the three lifts in 'powerlifting', a sport where people compete on how much barbell weight they can lift. The other two are the bench press and the deadlift. Under international rules, the lifter must sit down until the top of the thigh at the hip joint is lower than the top of the knee.
A common myth
Do squats also build the back of the thigh?
The muscles on the back of the thigh (the hamstrings) do work during a squat. But in a study where people who had not been lifting weights squatted for several weeks, the front of the thigh and the buttocks grew while the back of the thigh barely grew. In another study that compared squat depths, the back of the thigh did not change either.
Summary
Sit down, stand up, and use the leg muscles together
A squat is an exercise where you bend your knees and hips to sit down from standing and stand back up, using the front of the thigh, the buttocks and the inner thigh together. But in studies where people only squatted for several weeks, the back of the thigh barely grew. Related terms: compound exercise, range of motion, powerlifting.