What is the cake test? Easy: geographic data, or a map, is open only if someone can make you a gift of a cake with your map on it.
The cake test is inspired by the dissident test and the desert island test used by the Debian community to gauge software freedom for packages to be included in a free and open distribution.
For data to pass the cake test, you must be able to freely share the data with someone (the baker) who can re-use it for a profitable activity (the baking of cakes) and is then freely able to redistribute the resulting derived work (the cake).
The cake test can apply to all kinds of information resources, not just geodata. A resource that passes the cake test will be open in the sense of the Open Knowlege Definition. You could print a research paper onto a cake, a chart based on a dataset, some code describing an algorithm. Obviously a map just looks prettier on a cake.
The objective of the Cake Test is quite simple:
If a layperson can’t decide if one can or cannot give away a cake, or cannot do this easily, then the data or the maps cannot be freely used.
And you could be sure that if two datasets each passed the cake test, then it should be fine to give someone a cake decorated with parts of both of them – that is the intention of the data makers.