/ Dictionary / Index A actin: Meaning and Definition of Find definitions for: ac•tin Pronunciation: (ak'tun), [key] — n. Biochem. a globulin that is present in muscle plasma and that in connection with myosin plays an important role in muscle contraction. actin- Pronunciation: [key] var. ofbefore a vowel: actinism. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: actin (Thesaurus) ACTH actinal Related Content Daily Word Quiz: verbose Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language