Musculus Deltoideus, also known as the deltoid muscle, is a triangular-shaped muscle located in the shoulder region. It is responsible for the movement and stabilization of the shoulder joint. There are a number of synonyms for this muscle including deltoid, colliculus humeri, acromial muscle, triangle muscle, triangular muscle, and delta-shaped muscle. The deltoid muscle is composed of three distinct fibers (anterior, middle, and posterior) which all contribute to the movements of the shoulder joint. By developing the deltoid muscles through targeted exercise, individuals can improve their overall shoulder strength and functionality.