A ghost slip or stub is the term applied to any motorway sliproad (exit) that was only ever half-built, usually leaving a couple of small stubs in the hard shoulder. A ghost slip is the most obvious sign of a half-built service area, but it can also mark a junction which was never built.
Wherever possible, all the necessary ghost slips are included in the original construction of any road, as it makes it much easier to build the connection. However, unnecessary ghost slips ought to be avoided as they are often miss-used as parking places.
This shouldn't be mixed up with police patrol ramps, which are similar in appearance but usually at a higher level than the rest of the road and signed as such. These were purpose-built for police cars to watch traffic, but are rarely used these days.