revolving door

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A door, especially at the entrance of a building, typically made of three or four rigid upright sections joined at right angles and rotating about a central upright pivot.
• noun A recurring pattern of events or problems.
• noun A situation in which people remain or work only a short time before going elsewhere.
• noun A situation in which people with experience in an industry take government jobs in agencies that set policy for that industry and in which government employees take private-sector jobs in order to use their connections and knowledge to favorably influence government policy regarding their industry.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A door that rotates around a central pivot.
• noun figuratively A system or institution in which people exit and immediately reenter.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a door consisting of four orthogonal partitions that rotate about a central pivot; a door designed to equalize the air pressure in tall buildings
• noun an organization or institution with a high rate of turnover of personnel or membership