One who spends entirely too much time partaking in muscle building, strength training, cardiovascular, or aerobic activity. Specifically, one who does so at a health club or gym.

Often used derrogatorily by people who do not partake in or understand this lifestyle, some self-proclaimed gym rats use the term as a status symbol or for positive reinforcement.

Rare: The term is sometimes used with homosexual or dim-witted connotations, but both are entirely unfounded (the author of this definition is a gym rat, and does not fit these connotations).

In terms of sports, if someone is called a Gym rat it is to be taken positively. Someone who puts in the work more than anyone else to become a better player, is considered a gym rat.

Some one who goes to the gym everyday

Some one who continually post statuses on their facebook that they are at the gym

Someone who spends all their time in a gym but does absolutely zero excercise.