The medical term thrombus arises from the Greek [θρόμβος] (pronounced thrombos) meaning "a lump", "a piece of milk curd", or a "clot". This Greek term was later adopted in Latin [thrombus] and is used in this unchanged form today. The plural for thrombus is [thrombi].  A synonymous Latin term is [coagulum] (pl. coagula).

In medicine, a thrombus is a blood clot that has the connotation of being stationary, fixed to a vessel wall. If this blood clot were to be freed and start moving with the blood flow, then the clot should be called an "embolus" (Lat; pl. emboli).

Because of its anatomical characteristics. in the presence of atrial fibrillation, the left atrial appendage is particularly prone to the formation of thrombi, which can embolize and cause brain strokes.

The root term for this word is [-thromb-]. Examples of its use are:

• Thrombosis:  The suffix [-osis] means "condition" with the connotation of "many". A condition of multiple thrombi
• Thrombocytopenia: A combination of root terms; the root term  [-cyt-] means "cell" and the suffix [-(o)penia] means "a deficiency". A deficiency of platelets (the "clotting" cells)

Sources:
1. "The Origin of Medical Terms" Skinner, HA 1970 Hafner Publishing Co.
2. "Medical Meanings - A Glossary of Word Origins" Haubrich, WD. ACP Philadelphia