The exercise where you step forward and lower your back knee toward the floor is called a lunge.
In a forward lunge, one leg moves ahead and bends while the rear leg extends behind, with the back knee lowering toward the ground. The front foot generally remains flat, and the movement is reversed by pushing through the legs to return to standing. Lunges can be performed with body weight or with dumbbells, kettlebells, or a barbell.
The exercise trains several lower-body muscle groups, especially the quadriceps, gluteus maximus, and hamstrings. A longer stride tends to emphasize the glutes, while a shorter stride places more emphasis on the quadriceps. Because the rear leg also contributes to the movement, a lunge is not exactly the same as a split squat, where the feet stay in place.
“Lunge” is a broad family name rather than a single fixed technique. Variations include reverse lunges, walking lunges, lateral lunges, and jumping or plyometric lunges. In yoga, related positions appear in poses such as the low lunge and warrior poses, but those are not automatically identical to a standard strength-training lunge.