A quad is a rectangular shaped area, especially on a university campus, that's enclosed by buildings. Your college friend might tell you to meet her in the quad after lunch.
Quad is an abbreviation, most commonly short for quadrangle, a kind of four-sided courtyard usually defined by a large lawn and surrounded by buildings. Another kind of quad — also an abbreviation — is the large muscle in the top of your thigh, more formally known as a quadriceps muscle. Quadrangle and its nickname, quad, were 1800's Oxford slang, from the Latin root quadri, or "four."