The Collaborative International Dictionary of English v.0.48:

Embolus \Em"bo*lus\, n.; pl. Emboli. [L., fr. Gr. ? pointed so
   as to be put or thrust in, fr. ? to throw, thrust, or put in.
   See Emblem.]
   1. Something inserted, as a wedge; the piston or sucker of a
      pump or syringe.
      [1913 Webster]

   2. (Med.) A plug of some substance lodged in a blood vessel,
      being brought thither by the blood current. It consists
      most frequently of a clot of fibrin, a detached shred of a
      morbid growth, a globule of fat, or a microscopic
      organism.
      [1913 Webster]

WordNet (r) 3.0 (2006):

embolus
    n 1: an abnormal particle (e.g. an air bubble or part of a clot)
         circulating in the blood