Origins
The prefix 'de-' is a major Latin-derived prefix in English, used to form verbs and adjectives indicating removal, reversal, downward motion, separation, or sometimes intensification. It descends from Latin 'de,' a preposition and prefix meaning 'down from,' 'from,' 'about,' or 'concerning.' The Proto-Indo-European ancestor is usually reconstructed as *de, a demonstrative or directional particle, but the prehistory is imperfect.
In Latin, 'de-' formed verbs across a wide semantic range: downward motion ('descendere,' to descend), removal ('deponere,' to put down / depose), completion ('definire,' to define / bound completely), intensification ('demandare,' to commit / demand), and reversal or undoing. This semantic versatility is why English 'de-' words span such a wide range of meanings.
Latin 'de-' gave rise to two related prefix lines in English. First, 'de-' itself was borrowed directly through Anglo-Norman French and Latin. Second, Latin 'dis-' (a closely related but distinct prefix meaning 'apart, asunder') passed through Old French as 'des-' and eventually became English 'dis-.' French words that had 'des-' entered English either as 'de-' or 'dis-,' depending on when they were borrowed. This is why English has pairs like 'deform' / 'disfigure,' 'defeat' / 'defy,' all tracing to the same extended family of Latin removal-prefixes.
Middle English
In Middle English (c. 1200–1500), 'de-' arrived in hundreds of borrowed words: 'decide' (cut off), 'declare' (make clear), 'decline' (bend down), 'declare,' 'defeat' (undo), 'defend' (ward off), 'defer' (carry away), 'define' (bound, limit), 'delay' (leave aside), 'deliver' (set free), 'demand,' 'demolish,' 'deny,' 'depart,' 'depend,' 'describe,' 'deserve,' 'desire,' 'despair,' 'destroy,' 'detain,' 'determine,' 'develop,' 'devote,' 'devour.' In most of these the prefix was not clearly analysable to English speakers because both the prefix and the base arrived together.
In Early Modern and Modern English (c. 1500–present), 'de-' became a productive prefix that speakers could apply to native bases, especially to form verbs meaning 'undo' or 'remove.' 'Debunk' (1923, American), 'defuse' (1943, military), 'deice' (1918, aviation), 'deplane' (1923, aviation), 'decaffeinate,' 'defrost,' 'deforest,' 'devalue,' 'destabilise' — all show 'de-' freely attached to native or naturalised bases.
The productive Modern English 'de-' has a reliable reversative meaning: to undo or remove the base. 'Deactivate' (make no longer active), 'decompose' (break down), 'declassify' (remove secrecy), 'decontaminate' (remove contamination), 'decriminalise' (remove criminal status), 'decentralise' (undo centralisation), 'dehydrate' (remove water), 'deregulate' (remove regulation), 'devalue,' 'defrost,' 'defog,' 'deforest,' 'desalinate.' In technical and scientific registers this productive 'de-' has produced hundreds of coinages in the twentieth century, especially in chemistry and computing: 'deallocate,' 'deserialize,' 'decompile,' 'debug,' 'decrypt,' 'deduplicate.'
Latin Roots
Allomorphy with 'de-' is minimal. It does not generally assimilate to the following consonant, unlike Latin 'in-.' The vowel sometimes varies between /dɪ/, /di/, and /diː/ depending on stress and transparency, the same pattern as 're-': naturalised words ('deceive,' 'decide,' 'deduce') use /dɪ/, while live, analysable coinages ('de-ice,' 'de-escalate,' 'de-stress') use /diː/. Hyphenation often signals this productive, live use.
'De-' is sometimes confused with 'dis-.' The two share a distant ancestor (Latin 'dis-' probably from PIE *dwis, 'apart,' though some scholars argue 'de-' and 'dis-' are not directly related despite Old French 'des-' blurring them), and in many pairs they feel interchangeable but are not. 'Deforest' and 'disafforest' both exist; 'defeat' and 'disaffect' are cousins. In general, 'dis-' negates or separates ('dislike,' 'disagree,' 'disappear'), while 'de-' reverses or removes from ('deforest,' 'defrost,' 'decode').
A few Modern English 'de-' words carry older intensifying or completive senses: 'declare' (make fully clear), 'demand' (fully require), 'deliberate' (weigh fully), 'denounce' (fully announce / announce against). These are Latin fossils where 'de-' means 'completely' or 'down to the bottom' rather than 'remove.' Native speakers rarely notice this and parse them as unanalysable wholes.
Legacy
Representative 'de-' words include: debate, debut, decay, decease, deceive, decide, declare, decline, decorate, decrease, deduct, defeat, defend, defer, define, defy, degree, delay, delete, deliver, demand, demolish, deny, depart, depend, deploy, deposit, depress, descend, describe, desert, design, desire, despair, despise, destroy, detain, detect, determine, detest, detour, develop, devote. Reversative/productive: deactivate, debug, decaffeinate, declassify, decode, decompose, decompress, decontaminate, decrypt, dedramatise, defrost, dehydrate, deice, demist, demystify, denationalise, depopulate, deregulate, desalinate, destabilise, devalue.