Monopod
A single rear leg or post under the buttstock that supplies steady elevation support for a prone shot while the bipod carries the front of the rifle.
A monopod is a single adjustable leg or post mounted under the toe of the buttstock, used to carry the rear of the rifle while a bipod carries the front. Its job is purely to set and hold elevation: by extending or retracting the post the shooter can park the reticle on target and free the support hand from squeezing a bag. In the prone position this gives a stable two-point rest that holds elevation without constant muscle effort.
A monopod is one of several ways to manage the rear of the gun, the others being a squeezed rear bag or the heel of the support hand. Each has trade-offs in speed and fine control, and many shooters combine a monopod with loading the bipod up front to build a position that tracks straight back under recoil and returns to point of aim.