/əˈfɛkt/
[verb] To have an influence on someone or something, causing a change or producing an effect. It can also mean to touch someone's feelings or emotions in a significant way.
The word 'affect' secretly carries the idea of being physically seized — as if emotions literally grab hold of you.
The story of 'affect' begins in ancient Rome, with the Latin verb 'facere', meaning 'to do' or 'to make'. This was one of the most productive roots in all of Latin, spawning hundreds of descendants across European languages. When Latin speakers wanted to add the sense of 'doing something toward' or 'acting upon', they attached the prefix 'ad-' (meaning 'to' or 'toward') to 'facere', producing 'adficere', which later smoothed into 'afficere'. This verb carried the vivid sense of working upon something, acting on it directly — almost physically seizing or striking it.
From 'afficere' came the past participle form 'affectus', meaning 'acted upon' or 'influenced'. Latin writers used 'affectus' to describe states of mind and body that had been shaped by outside forces — an emotion, a condition, a frame of mind produced by something acting on you. The noun 'affectus' described feelings and moods in this way. This Latin root then passed into Old French as 'affecter', keeping the core sense of influencing or working upon something, before crossing the Channel into English.
English adopted 'affect' in the late 14th century, likely drawing directly from both Old French 'affecter' and from Latin scholarly texts, which educated writers of the period read fluently. Early English uses tended to follow closely in the Latin tradition — 'to affect' something meant to act upon it, to have an effect on it. Medical and philosophical writers of the period used it constantly, because they needed precise language for describing how one thing influenced another: how cold air affects the lungs, or how grief affects the mind.
Over the following centuries, 'affect' developed an interesting second life in English as a word about emotions specifically. By the 17th century, scholars in psychology and philosophy were using 'affect' as a noun meaning a feeling or emotional state — a direct echo of the old Latin 'affectus'. This technical, noun form is still very much alive today in psychology, where clinicians speak of a patient's 'affect' meaning their outward expression of inner feeling. In this way, the word doubled back on itself: it originally meant to act upon emotions, and then came to describe emotions themselves.
One of the most persistent confusions in English is the difference between 'affect' and 'effect'. The two words are close cousins, both descending from Latin 'facere' through different prefixes — 'affect' from 'ad-' (toward) and 'effect' from 'ex-' (out), giving Latin 'efficere', meaning to bring something out or accomplish it. So while 'affect' is the verb describing the action of influencing, 'effect' is typically the noun describing the result. The chain runs: Latin 'ad-' + 'facere' → 'afficere' → 'affectus' → Old French 'affecter' → English 'affect'; and separately, Latin 'ex-' + 'facere' → 'efficere' → 'effectus' → Old French 'effect' → English 'effect'. Two branches from the same ancient root, tangled together ever since.