Thes·sa·ly (thěs'ə-lē) A region of east-central Greece between the Pindus Mountains and the Aegean Sea. Settled before 1000 B.C., it reached the height of its power in the sixth century B.C. but soon declined because of internal conflicts. Thes·sa'lian (thě-sā'lē-ən, -sāl'yən), Thes'sa·lo'ni·an (-lō'nē-ən) adj. & n.