Real Dictionary

muscle
a protein that participates in many kinds of cell movement, including muscle contraction, during which it interacts with filaments of a second protein, myosin [...]
a tissue composed of bundles of elongated cells capable of contraction and relaxation to produce movement in an organ or part [...]
the act or process of regenerating or the state of being regenerated; rebirth or renewal [...]
the act, process, or result of differentiating [...]
uncountable noun: If you say that someone has muscle, you mean that they have power and influence, which enables them to do difficult things.
variable noun: A muscle is a piece of tissue inside your body which connects two bones and which you use when you make a movement.