Definition: The word "antiplatelet" comes from Greek, meaning "against platelets." It refers to a substance that acts as an antagonist or counteracting agent against platelets, which are small blood vessels that help to clot blood in areas prone to bleeding. Definition: A substance used to inhibit the action of platelets (small blood cells) and thus reduce their ability to break down blood vessels. Antiplatelet agents can be either vitamin K antagonists, which interfere with platelet aggregation or in thrombocytopenia inhibitors, which stop the production of platelets. For example, Warfarin is an antiplatelet medication used to prevent excessive bleeding from forming when you're taking aspirin because it inhibits the activity of clotting factors.