Q: Lens? Window?
A: The clear material in front of a flashlight's light source and optical system, intended to protect them, is usually called a lens. However, it is more accurate to call it a "window," as a lens could also refer to some sort of optical system where light goes through a piece of clear glass or plastic and is distorted in some way (e.g. collimated or dispersed). Technically, a window is nothing but a plano-plano lens (flat on both sides), but it's easiest and clearest to call it a window and save "lens" for the above-mentioned optical system.