1US. A person employed to mediate between citizens and local government or other official agencies. Also: a person who represents the interests of (a section of) the community to a policy-making body, or who campaigns for a particular cause; a member of a pressure group, a lobbyist.

2Originally US. A volunteer who provides help to disabled, elderly, or vulnerable members of the community; especially one trained to represent to the relevant authorities the interests of a person with learning difficulties or other cognitive impairment, and to provide help with administrative responsibilities.