a group of senior members of the political party that is out of power; these members would probably assume corresponding positions as ministers in the British Cabinet if their party was elected
The leadership of the opposition, poised to take the reins of government and its ministries in the event elections are called and lost by the ruling majority party. Shadow cabinets operate in a manner akin to a government in exile, formulating policies they are not empowered to enact -- but that might become law if they were elected.
A collective term for the critics in each of the opposition parties, particularly those in the official opposition, who might comprise the cabinet should the party come to power. These critics scrutinize the programs and policies of the government department to which they are assigned.