Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

thrombus
     n : a blood clot formed within a blood vessel and remaining
         attached to its place of origin
     [also: {thrombi} (pl)]