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'''Borough''' referred to an administrative division in the British government with the right to elect a member to [[Parliament]].

'''Rotten borough''' referred to a burough whose population had declined over the centuries to the point that only a handful of residents remained, yet still retained the right to elect an MP.

'''Pocket borough''' referred to a Rotten borough whose residents were the clients of a (typically) aristocratic patron.  The patron thus had a pariamentary seat "in his pocket", that is the ability to fill it as he saw fit.  Jack's cousin Edward Norton owned the pocket borough of Milport, which was filled first by Jack's father, General Aubrey, and later by Jack.

'''The Reform Act of 1832''' effectively abolished these practices, as it disenfranchised most Rotten Buroughs and distributed their representation to larger population centers.