1. Watery proteinaceous portion of the blood that remains after clotting. Synonym: blood serum
  2. Clear watery fluid especially that moistening the surface of serous membranes or that exuded through inflammation of any of these membranes.
PAC, 2004, 76, 1033 (Glossary of terms used in toxicokinetics (IUPAC Recommendations 2003)) on page 1070