In the Welsh language, caer means "fortress, fort, citadel, castle".
Caer is the Welsh name for the city of Chester, situated in northwest England. It also forms, as a prefix, the Welsh equivalent of -caster, -cester and -chester in place names. The English word derives from Latin castrum "fortified post", more common in the plural castra meaning "military camp", and is the equivalent of castell "castle" in Welsh. The word caer itself derives from the Brittonic word *kagro-, as does the word cae (modern Welsh for "field", i.e. an enclosed piece of land).
Examples in Wales include:
- Caerdydd (originally Caerdyf) - Fortress of (the river) Taf;
- Caergybi - Fortress of (Saint) Cybi;
- Caernarfon - Fortress in Arfon;
- Caerffili - Fortress of Ffilli.
- Caerwent - fortress of Gwent.
Examples of Welsh "Caer" names as exonyms for English cities:
- Caerfuddai - Chichester;
- Caergaint - Canterbury;
- Caergrawnt - Cambridge;
- Caerhirfryn - Lancaster;
- Caerliwelydd - Carlisle;
- Caerloyw - Gloucester;
- Caerlwytgoed - Lichfield;
- Caerlŷr - Leicester;
- Caersallog - Salisbury;
- Caerwrangon - Worcester;
- Caerwynt - Winchester;
- Caerwysg - Exeter
Southern Scotland contains many placenames with a "Caer" element, toponymic remnants of the kingdoms of the Hen Ogledd. Some examples are:
- Carriden: fort of Eidyn. Falkirk
- Carruthers: South West Clan named for a Fort now in ruins.
- Caerlaverock: fort of Llywarch. Dumfriesshire
- Cramond: Caer Almond, Fort on the river Almond. Edinburgh
- Caerlanrig: fort in the glade/clearing. Roxburghshire
- Carfrae: fort on the brae. East Lothian and Berwickshire
- Cardrona: fort of Ronan. Peeblesshire
- Geiriadur Prifysgol Cymru, vol. 1, p. 384.