Definition · Plain-language
Prefix
A prefix is an affix added to the beginning of a word or root to change its meaning, such as un- in unhappy or re- in rewrite.
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How prefixes change meaning
A prefix attaches to the front of a base word or root and modifies its meaning, usually without changing its part of speech. Unlike many suffixes, a prefix rarely shifts the word class: unhappy is still an adjective, just as happy is. Prefixes often signal negation or opposition (un-, in-, dis-, non-), repetition (re-), degree or size (over-, under-, mega-), time or order (pre-, post-, ex-), or position (sub-, inter-, trans-). Knowing a handful of common prefixes lets a reader unlock the meaning of many unfamiliar words — recognising that pre- means before makes preview, predict and prehistoric immediately transparent.
Prefixes from Latin and Greek
A large share of English prefixes come from Latin and Greek, especially in academic and scientific vocabulary. Latin gives us sub- (under), trans- (across), inter- (between) and circum- (around); Greek gives us anti- (against), hyper- (over), auto- (self) and tele- (far). These combine productively: a telescope lets you see far, an autobiography is a self-written life, and an antibiotic works against living organisms. Because the same prefix recurs across hundreds of words, learning its meaning multiplies a reader’s vocabulary far more efficiently than memorising words one at a time.
Spelling and hyphenation of prefixes
Some prefixes change their spelling to match the root they join — a process called assimilation. The Latin prefix in- (not) becomes im- before b, m or p (impossible, immature), il- before l (illegal) and ir- before r (irregular). Hyphenation varies: most prefixes attach directly (rebuild, unhelpful), but a hyphen is often kept to avoid a doubled or awkward letter sequence (re-enter, co-operate in British usage), before a proper noun (anti-British), or to prevent confusion between words (re-cover a chair versus recover from illness). A current dictionary records the accepted spelling for any particular case.
Key facts
At a glance
- Definition: letters added to the start of a word to change its meaning
- Position: precedes the root (contrast: suffix follows it)
- Category: a type of affix
- Usually keeps: the original part of speech (unhappy is still an adjective)
- Common senses: negation (un-), repetition (re-), time (pre-, post-)
- Example: un- + happy → unhappy; re- + write → rewrite
Common misconceptions
What people often get wrong
Often heard: A prefix changes the part of speech of a word.
Actually: Prefixes usually leave the part of speech unchanged — unhappy stays an adjective like happy. It is suffixes that more often shift the word class. Prefixes mainly change meaning.
Often heard: Prefixes always attach to the front without any spelling change.
Actually: Many prefixes assimilate to the root: in- becomes im- before p (impossible), il- before l (illegal), ir- before r (irregular). The spelling adapts to ease pronunciation.
Often heard: Any letters at the start of a word form a prefix.
Actually: A true prefix is a meaningful unit you can remove to leave a recognisable root. The re- in rewrite is a prefix; the re- in red is simply part of the word.
Going deeper
