A fitness company founded by Greg Glassman in 2000. CrossFit's exercise program is practiced by members of approximately 4,350 affiliated gyms, most of which are located in the United States, and by individuals who complete workouts that the company constructs.

CrossFit describes its strength and conditioning program as “constantly varied, high intensity, functional movement," with the stated goal of improving fitness (and therefore general physical preparedness), which it defines as "work capacity across broad time and modal domains." Workouts are typically short—20 minutes or less—and intense, demanding all-out physical exertion. They combine movements such as sprinting, rowing, jumping rope, climbing rope, flipping tires, weightlifting, carrying heavy objects, and many bodyweight exercises; equipment used includes barbells, dumbbells, gymnastics rings, pull-up bars, kettlebells, medicine balls, and boxes for box jumps. These elements are mixed in numerous combinations to form prescribed "Workouts of the Day" or "WODs". Hour-long classes at affiliated gyms, or "boxes," typically include a warm-up, a skill development segment, the high-intensity WOD, and a period of individual or group stretching. Performance on each WOD is often scored and/or ranked to encourage competition and to track individual progress. Some affiliates offer additional classes, such as Olympic weightlifting, which are not centered around a WOD.