Which of the following muscles is named after its shape?

The muscle that is named after its shape is deltoid. The term "deltoid" comes from the Greek letter delta (Δ), which has a triangular shape, mirroring the muscle's broad, triangular appearance over the shoulder. The deltoid muscle forms the rounded contour of the shoulder and is responsible for major movements of the arm, particularly abduction.

Orbicularis refers to a circular muscle, such as the orbicularis oculi around the eyes, but it doesn't specifically convey a unique shape like the deltoid does. Similarly, procerus describes a muscle that may have some descriptive qualities related to its location and function but does not highlight its shape as distinctly as deltoid. Rectus denotes a straight orientation in muscles, often used in the context of muscles that run parallel to the midline of the body, rather than being named solely for their shape.