In railroad terminology, a double slip is a piece of trackwork, superficially representing a diamond crossing (two tracks crossing over each other at a angle of about 10° to 30° or so.) Unlike a simple crossing, however, a double slip allows the train to take either of the routes on the opposite side of the crossing in all directions - approaching the double slip in any direction, a train can take either the straight path that crosses the other straight path, or a curved path that curves onto the other line.
A double slip is set up so that it has two positions it can be switched between; both crossovers selected, or both curved, diverging routes selected.
Functionally, a double slip is equivalent to two turnouts (switches, or in British parlance, points) arranged back to back. Now, a regular turnout is much simpler to construct (being pretty much a stock item, assembled out of standard components to a standard plan) and simpler to maintain, also; so a double slip is only used when it has to be, generally because of a restricted area available, since the double slip turnout is much shorter than those two back-to-back regular turnouts. Double slips are utilized in confined spaces, most commonly passenger terminals and other such places where a lot of tracks and a lot of desired routes exist.
There are two designs of double slip turnouts in use worldwide. The regular pattern, sometimes called the English pattern, has the switch blades (a double slip has eight of them, while a turnout has but two) mounted inboard of the crossing frogs, while the Baeseler pattern, used only in German-speaking nations to my knowledge, has the switch blades mounted outboard of the frog. This latter pattern means that firstly the moving hardware is less bunched together in the center of the design, and is possibly somewhat more robust, and secondly that the radius of the curved path is greater, meaning for a smoother transition. The Baeseler design can be easily distinguished because on it, the rails of the curved paths never cross, while on the English pattern, the two curved paths overlap one another.
Like all custom and complicated trackwork, the double slip (and its cousin the single slip) are tolerated as a necessary evil rather than loved, and if the possibility of replacing them with simpler and more standard trackwork is available, that work will be done.