Both words name the same thing: a reference work with articles on a wide range of subjects, usually arranged alphabetically. Encyclopedia is the American spelling; encyclopaedia is the British one, closer to the original Greek-derived form. Neither spelling is more correct — it depends on which English you are writing.
The core difference
One reference work, two regional spellings.
- encyclopedia — American spelling: She looked up the country's history in an old encyclopedia.
- encyclopaedia — British spelling of the same word: The old encyclopaedia set filled an entire bookshelf.
How to tell them apart
The word traces back to New Latin encyclopaedia, itself from the Ancient Greek phrase for a general or well-rounded education. British spelling keeps that fuller ae form, while American spelling simplifies it to e — the same pattern that separates paediatric from pediatric.
A handy memory trick: the British spelling is the longer, older-looking one, so think of it as keeping the classical roots intact. When editing for a British audience, watch for the American form creeping in from spell-checkers set to US English.
