muscle
n. C / Un. a part of your body that helps you move. You use them to lift things, walk, or even make your heart beat.
n. a tissue composed of cells or fibers, the contraction of which produces movement in the body. Often used metaphorically to refer to physical power or political influence.
He works out every day to build more muscle.
The athlete pulled a muscle in her leg during the final sprint of the race.
The heart is a unique type of involuntary muscle that must function continuously throughout a person's life without the benefit of conscious control or rest.
From Middle English muscle, muscule, muskylle, and in part from Middle French muscle, from Latin mūsculus (“a muscle”, literally “little mouse”) because of the mouselike appearance of some muscles, from mūs (“mouse”). Doublet of mussel. More at mouse.
Uncountable when referring to the mass of tissue in the body; countable when referring to a specific organ or distinct body part.