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

Moby Thesaurus II by Grady Ward, 1.0:

143 Moby Thesaurus words for "thrombosis":
   Jacksonian epilepsy, Rolandic epilepsy, abdominal epilepsy, access,
   acquired epilepsy, activated epilepsy, affect epilepsy,
   akinetic epilepsy, angina, angina pectoris, aortic insufficiency,
   aortic stenosis, apoplectic stroke, apoplexy, arrest, arrhythmia,
   arteriosclerosis, atherosclerosis, atrial fibrillation, attack,
   auricular fibrillation, autonomic epilepsy, beriberi heart,
   blockage, cardiac arrest, cardiac epilepsy, cardiac insufficiency,
   cardiac shock, cardiac stenosis, cardiac thrombosis, carditis,
   clonic spasm, clonus, congenital heart disease, convulsion,
   cor biloculare, cor juvenum, cor triatriatum, coronary,
   coronary insufficiency, coronary thrombosis, cortical epilepsy,
   cramp, cursive epilepsy, diastolic hypertension, diurnal epilepsy,
   eclampsia, encased heart, endocarditis, epilepsia,
   epilepsia gravior, epilepsia major, epilepsia minor,
   epilepsia mitior, epilepsia nutans, epilepsia tarda, epilepsy,
   extrasystole, falling sickness, fatty heart, fibroid heart, fit,
   flask-shaped heart, focal epilepsy, frenzy, frosted heart,
   grand mal, hairy heart, haute mal, heart attack, heart block,
   heart condition, heart disease, heart failure, high blood pressure,
   hypertension, hypertensive heart disease, hysterical epilepsy,
   ictus, ischemic heart disease, larval epilepsy, laryngeal epilepsy,
   laryngospasm, latent epilepsy, lockjaw, matutinal epilepsy,
   menstrual epilepsy, mitral insufficiency, mitral stenosis,
   musicogenic epilepsy, myocardial infarction,
   myocardial insufficiency, myocarditis, myoclonous epilepsy,
   myovascular insufficiency, nocturnal epilepsy, occlusion, ox heart,
   palpitation, paralytic stroke, paroxysm, paroxysmal tachycardia,
   pericarditis, petit mal, physiologic epilepsy, pile,
   premature beat, pseudoaortic insufficiency, psychic epilepsy,
   psychomotor epilepsy, pulmonary insufficiency, pulmonary stenosis,
   reflex epilepsy, rheumatic heart disease, rotatoria, round heart,
   sclerosis, seizure, sensory epilepsy, serial epilepsy, spasm,
   stony heart, stoppage, stroke, tachycardia, tardy epilepsy,
   tetanus, tetany, throes, thromboembolism, tonic epilepsy,
   tonic spasm, torsion spasm, traumatic epilepsy,
   tricuspid insufficiency, tricuspid stenosis, trismus, turtle heart,
   ucinate epilepsy, varicose veins, varix, ventricular fibrillation,
   visitation