An adapter for releasably fastening a portable/compact unit or object to a holding device. The adapter comprises a base plate and two projections projecting away from one another in opposite directions and at a spacing from the base plate. The base plate has means for attaching the adapter to the unit or object or attached fixedly thereto or is configured in one piece with it. A guide disc is mounted on the base plate, is parallel to said base plate and can be rotated at least to a limited extent between at least one released position and at least one locked position. The guide disc has a middle opening, into which the two projections extend. The opening comprises two plug-in regions into which a holding device suitable for interaction with the adapter can be plugged in at least partially when the guide disc is situated in a released position.