Dorchester was a parliamentary constituency centred on the town of Dorchester in Dorset. It returned two Members of Parliament to the House of Commons of the Parliament of the United Kingdom from 1295 to 1868, when its representation was reduced one member.
The constituency was abolished by the Redistribution of Seats Act 1885, after which Dorchester was placed in the new Dorset South constituency. In 1918 it was transferred to Dorset West, where it has remained since.
References
Dorchester (UK Parliament constituency) Wikipedia(Text) CC BY-SA