There is no difference in meaning — only in spelling by region. <em>Defence</em> is standard in Britain, Ireland, Australia, New Zealand and Canada ("the case for the defence"); <em>defense</em> is standard in the United States ("the Department of Defense"). Whichever you choose, the derived words take an <strong>s</strong> everywhere: <em>defensive</em>, <em>defensible</em>, <em>defensively</em>.
The core difference: which side of the Atlantic
This belongs to the same family as licence/license and offence/offense — British English keeps the -ce ending, American English uses -se. Both spellings cover every sense of the word: military defence, legal defence, sporting defence, self-defence, a defence of an argument.
Canada follows the British form here, which surprises people who expect Canadian spelling to track American practice.
The test you can run
Look at the rest of your document. If you are already writing colour, organise and centre, write defence. If you are writing color, organize and center, write defense. Then set your spellchecker's language to match, because it is the only reliable way to catch the odd one that slips through.
The derived forms are the real trap
Here is where British writers actually go wrong. The -ce ending does not carry through to the adjective and its relatives. Every variety of English writes:
- defensive, never defencive
- defensible, never defencible
- defensively, defensiveness
Only the noun itself changes. So a British writer produces a strong defence and a defensive posture in the same paragraph, which looks inconsistent but is entirely correct.
The words built directly on the noun do follow it: British defenceless, American defenseless; British self-defence, American self-defense.
The edge case: proper nouns keep their own spelling
Institution names are not spelled to suit your dialect. A British newspaper writes about the US Department of Defense with an s, and an American paper writes about the UK Ministry of Defence with a c. The same applies to job titles attached to them — the US Secretary of Defense, the UK Defence Secretary.
One difference this pair does not have
Be careful not to import the licence/license rule. In British English, licence is the noun and license the verb, and the same split applies to practice/practise and advice/advise. No such split exists for defence: it is a noun only, and there is no verb to defence. The verb is defend. So if a British text shows defense, it is simply American spelling, not a clever noun-verb distinction.
Frequently Asked Questions
Is "defense" a misspelling in British English?
It is not a misspelling in itself, just the American form. In a British document it will read as inconsistent, so use defence unless you are quoting an American source or naming a US institution.
Is it "defensive" or "defencive"?
Defensive, in every variety of English. The adjective always takes an s, even for writers who spell the noun defence.
Which spelling does Canada use?
Canada uses defence, following British practice, despite following American spelling for some other words.