Reflex comes from Latin reflexus ('a bending back'), past participle of reflectere ('to bend back'), from re- ('back') + flectere ('to bend'). The historical chain includes Latin re-, Latin reflectere, and Latin flectere. Older glosses in the chain include "a bending back", "to bend back", "back", and "to bend", which help explain the semantic range of the modern word. Morphologically, reflex is best read as compound and borrowing, yielding an English noun and adjective shaped by both form history and later usage. In current English it functions as a noun and adjective, with the sense "an automatic, involuntary response to a stimulus". English derivatives include reflexive, reflexology, and reflexly; related forms include reflect, reflection, flex, and flexible. The result is an English noun and adjective whose etymology preserves the older forms while supporting the word's specialized modern meaning.