Source : Webster's Revised Unabridged Dictionary (1913)

Thrombosis \Throm*bo"sis\, n. [NL. See {Thrombus}.] (Med.)
   The obstruction of a blood vessel by a clot formed at the
   site of obstruction; -- distinguished from embolism, which is
   produced by a clot or foreign body brought from a distance.
   -- {Throm*bot"ic}, a.

Source : WordNet®

thrombosis
     n : the formation or presence of a thrombus (a clot of
         coagulated blood attached at the site of its formation)
         in a blood vessel