The Collaborative International Dictionary of English v.0.48:

Thrombus \Throm"bus\, n.; pl. Thrombi. [NL., fr. Gr. ? a lump,
   a clot of blood.] (Med.)
   (a) A clot of blood formed of a passage of a vessel and
       remaining at the site of coagulation.
   (b) A tumor produced by the escape of blood into the
       subcutaneous cellular tissue.
       [1913 Webster]

WordNet (r) 3.0 (2006):

thrombus
    n 1: a blood clot formed within a blood vessel and remaining
         attached to its place of origin