/ Dictionary / Index P prothrombin: Meaning and Definition of Find definitions for: pro•throm•bin Pronunciation: (prō-throm'bin), [key] — n. Biochem. a plasma protein involved in blood coagulation that on activation by factors in the plasma is converted to thrombin. Also called Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: prothrombin (Thesaurus) prothorax protist Related Content Daily Word Quiz: feline Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language