A revolving door is a building entryway consisting of multiple fixed doors mounted on a pivot so that at least one door always prevents drafts. Revolving doors are often used as metaphors.
A revolving door is mounted inside a rounded opening so that it fits perfectly with no gaps to allow air exchange from inside and outside. Someone who is not accustomed to a revolving door may push it completely around, finding himself at the starting point again. This occurrence has led to the use of the term revolving door to describe places with high employee turnover; the employee walks into a job and finds himself right back on the street shortly after. It's also used to describe the system in Washington, D.C., whereby regulators and legislators, upon retirement or being voted out, take positions with D.C. lobbying firms or private businesses with whom they had dealings previously.