thrombus (n.)

before vowels thromb-, word-forming element meaning "blood clot," from combining form of Greek thrombos "clot of blood" (see thrombus).

1706, Modern Latin, from Greek thrombosis "a clumping or curdling" (from thrombousthai "become curdled or clotted," from thrombos "clot, curd, lump;" see thrombus) + -osis.