What is the meaning of the word "quad"?

Definitions:

In anatomy, is a common abbreviation for the quadriceps femoris, a group of four muscles located in the front of the thigh. NOUN
- After a long run, her quads were sore, indicating she had a good lower-body workout.
In educational settings, refers to a quadrangle, which is a central courtyard typically surrounded by buildings. NOUN
- Students gathered in the quad to enjoy the sunny weather between classes.
In recreational terminology, refers to an all-terrain vehicle with four wheels that is designed for off-road use. NOUN
- They rented quads to explore the rugged trails at the mountain resort.