/ˈflɛkʃən/
[noun] The act of bending a joint or a part of the body, such as when you bend your elbow or knee. It is also used more generally to describe any bending or curving movement.
Every time you bend your elbow, you are performing an action whose name comes from the Latin word for 'bow' — the same root that gave archers their weapon.
The story of 'flexion' begins in ancient Rome, with the Latin verb 'flectere', meaning 'to bend' or 'to curve'. This verb was deeply embedded in Roman life and language, used to describe everything from bending a branch to changing the direction of a road. From 'flectere', Latin formed the noun 'flexio' (genitive 'flexionis'), meaning simply 'a bending'. It is this noun that passed almost unchanged into English, arriving in the early 17th century as medical and scientific writers began building a formal vocabulary for describing the movement of the human body.
The journey from Latin into English was not entirely direct. French, as so often happened during and after the Renaissance, served as a stepping stone. The Latin 'flexio' became 'flexion' in Old French and Middle French, and English writers working in anatomy and natural philosophy borrowed it from both Latin texts and French usage. By the time it settled into English in the 1600s, its spelling and pronunciation were already close to what we use today.
What makes 'flectere' particularly rich as a root is how productive it proved across the languages of Europe. The same Latin verb gave us 'flex' (to bend a muscle), 'flexible' (capable of bending), 'deflect' (via Latin 'deflectere', meaning to bend away — 'de-' meaning away + 'flectere'), and 'reflect' (via Latin 'reflectere' — 're-' meaning back + 'flectere', so literally 'to bend back'). Even 'inflection', the grammatical term for the changing of a word's form, comes from the same root via Latin 'inflectere' — 'in-' meaning into or upon + 'flectere' — originally meaning to bend or modify a sound or form.
There is also a surprising connection to archery. The Latin 'flectere' is believed by some scholars to share a distant Indo-European ancestor with words related to bending and curving that appear across many languages, including the idea of a bow — the weapon — as a curved or bent object. While this connection is debated, it reminds us that in the ancient world, bending was not merely a bodily movement but a fundamental physical principle observed in tools, nature, and the human form alike.
In modern English, 'flexion' is used primarily in anatomy, physiotherapy, and sports science, where it describes the specific movement of decreasing the angle between two body parts — for example, bending the knee or curling the fingers. Its opposite is 'extension', from the Latin 'extendere', meaning to stretch out. Together, these two terms form the basic vocabulary of joint movement, and 'flexion' carries within it over two thousand years of careful observation of the human body in motion.