The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

thrombosis
    n 1: the formation or presence of a thrombus (a clot of
         coagulated blood attached at the site of its formation) in
         a blood vessel