The Collaborative International Dictionary of English v.0.48:

Serosity \Se*ros"i*ty\, n. [Cf. F. serosit['e]. See Serous.]
   1. The quality or state of being serous.
      [1913 Webster]

   2. (Physiol.) A thin watery animal fluid, as synovial fluid
      and pericardial fluid.
      [1913 Webster]