Origins
The word 'effect' entered English in the 14th century from Old French 'effect' (later 'effet'), fromβββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββ Latin 'effectus' (an accomplishment, a performance, a result), the past participle of 'efficere' (to bring about, to accomplish, to produce). The Latin verb is composed of 'ex-' (out, from) + 'facere' (to make, to do), from PIE *dΚ°ehβ- (to put, to place, to make). The literal sense is therefore 'a thing made out' β something brought forth into reality by an action.
Latin 'facere' is one of the most productive verb roots in the English vocabulary, though its descendants are so numerous and varied that the family resemblance is often invisible. Direct derivatives include 'fact' (a thing done), 'factor' (a doer, a maker), 'factory' (a place of making), 'faculty' (an ability to do), 'fashion' (a manner of doing), 'feasible' (doable), and 'feat' (a deed). Compound forms with prefixes are even more numerous: 'affect' (to do to), 'defect' (an undoing), 'perfect' (thoroughly done), 'infect' (to put into), 'confect' (to make together), 'office' (from 'opificium,' a doing of work), 'benefit' (from 'benefactum,' a good deed), 'sacrifice' (from 'sacrum facere,' to make sacred), and 'manufacture' (from 'manu factum,' made by hand).
The PIE root *dΚ°ehβ- (to put, to place) also produced, through the Germanic branch, English 'do' and 'deed' β making 'do' and 'fact' distant cousins, both meaning at root 'to put into action.' Through Greek, the same root produced 'thesis' (a placing), 'theme' (a thing placed down), 'apothecary' (a storeroom, a placing-away), and the '-theca' suffix in 'library' (bibliotheca). Through Sanskrit, it produced 'dhΔ-' (to put, to place), the root of 'dharma' (that which upholds, from the causative 'that which is placed firmly').
Latin Roots
The distinction between 'effect' and 'affect' has plagued English speakers for centuries, and for good reason: the two words are near-twins in sound and closely related in meaning. Both derive from Latin 'facere' β 'effect' from 'ex-' + 'facere' (to make out, to accomplish) and 'affect' from 'ad-' + 'facere' (to do to, to act upon). The general rule β 'affect' is usually a verb meaning 'to influence,' while 'effect' is usually a noun meaning 'a result' β holds in most contexts but breaks down in two important cases: 'to effect change' (verb, meaning to bring about) and 'flat affect' (noun, a psychological term for diminished emotional expression).
The phrase 'in effect' (meaning 'in operation' or 'essentially') preserves the oldest English sense of the word β the state of being accomplished or operative. 'To take effect' and 'to come into effect' likewise preserve this sense. The plural 'effects' meaning 'personal belongings' (as in 'personal effects') developed from the sense of 'things accomplished or acquired' β one's effects are the tangible results of one's life.
In philosophy, 'effect' is the counterpart of 'cause' β the foundational relationship that David Hume famously subjected to skeptical scrutiny. In physics, named effects (the Doppler effect, the photoelectric effect, the butterfly effect) use the word in its purest sense: a phenomenon that results from a specific cause. The word's journey from 'a thing accomplished' to 'a result of any cause' represents a broadening from human agency to universal causation.