A tiny muscle in the human body that stabilizes the stapes, which is a horseshoe-shaped bone in the inner ear. The purpose of the stapedius muscle is to damp the movement of the stapes as a mechanism to auto-protect the ear from excess volume levels. Note that this mechanism is in no way sufficient to protect the ear from high volume levels or from extended exposure to loud sound.
(Trivia: the stapes is the smallest and lightest bone in the human body, and, at one millimeter long, the stapedius is the human body’s smallest skeletal muscle.)
(More trivia: a similar muscle, the tensor tympani, reduces the level of sounds created by chewing.)