1. A protein present in all cells and in muscle tissue where it plays a role in contraction: Actin is one of a large group of nitrogenous organic compounds forming the thin filaments in muscle fibers that are pulled on by myosin cross-bridges to cause a muscle contraction.

Some bacteria form actin tails to use for motility.

3. Etymology: According to A Dictionary of Scientific Terms by I.F. Henderson (Isabella Ferguson) and W.D. Henderson; Edinburgh; Oliver and Boyd Publication, 1920; this word comes from Ancient Greek ἀκτίς, "ray", "a star-shaped spicule (zoology)" + the English chemical suffix -in, -ine.

All of the other "modern" medical dictionary sources that include this actin protein term do not make any references to any etymological origins.