There is no difference in meaning — only in spelling. Harbor is the American form (boats crowded the harbor at dawn); harbour is standard in Britain, Ireland, Australia, New Zealand and Canada (she still harbours a grudge about it). Both cover the noun, a sheltered stretch of water, and the verb, to shelter something or hold onto it.
The core difference
This is one of the large family of -our / -or pairs: colour/color, favour/favor, neighbour/neighbor, harbour/harbor. British English keeps the u; American English drops it. Nothing about the meaning or pronunciation changes.
Both senses, both spellings
- Noun, a sheltered anchorage: The fishing fleet came back into the harbour before the storm.
- Verb, to give shelter to: They were accused of harbouring a fugitive.
- Verb, figuratively, to hold onto a feeling or idea: He harboured doubts about the plan from the start.
- Verb, of a place holding something unwanted: Damp brickwork can harbour mould.
The test
Ask which spelling you use for colour. If you write colour, write harbour. If you write color, write harbor. The two follow each other without exception, so you only ever have to remember one decision.
Proper nouns never change
This is the trap worth flagging. Place names keep whatever spelling they were given, regardless of where you are writing:
- Pearl Harbor, Hawaii — always without the u, even in a British newspaper.
- Bar Harbor, Maine; Harbor Freight; New York Harbor.
- Darling Harbour, Sydney; Victoria Harbour, Hong Kong; Newcastle Harbour.
Writing "the attack on Pearl Harbour" is a genuine mistake, not a dialect choice — the name is fixed.
What about Canada?
Canadian English generally keeps the British -our spellings, and harbour is standard there. Canadian place names largely follow suit, though a few official names use Harbor. Australian and New Zealand English use harbour throughout.
The consistency rule
Whichever you choose, apply it across the whole document along with its siblings — do not write harbour in one paragraph and favor in the next. If you are writing for an international audience, pick the variety your publisher or employer uses and set your spellchecker to match, because this is exactly the kind of inconsistency a reader notices without being able to say why.
