They mean the same thing — the end of a period of validity. Expiry is the usual British form (expiry date), expiration the usual American one (expiration date). Neither is more correct; match your audience. Expiration has one extra sense in American use: the act of breathing out.
The core difference
A straightforward variety split with one extra meaning on one side.
- expiry — British English: expiry date, on expiry of the lease.
- expiration — American English: expiration date. Also the medical sense of breathing out.
The other sense
Expiration is used in physiology for exhaling — inspiration and expiration being the two phases of breathing. That older sense connects to the word's root: Latin exspirare, to breathe out, and thence to the sense of a final breath, and thence to anything coming to an end.
Best-before and use-by
Neither word is the regulated term on food packaging in the UK, where use by concerns safety and best before concerns quality. That distinction matters more than the expiry/expiration one.
Frequently asked questions
Is expiry or expiration correct?
Both. Expiry is standard in British English, expiration in American English.
Why does expiration also mean breathing out?
Because it comes from Latin exspirare, to breathe out — the sense of an ending developed from the idea of a final breath.
What is the difference between use by and best before?
Use by is about safety; best before is about quality. In the UK these are the regulated terms rather than expiry.