What is the most proper name for a two-part mount like the one depicted below: The left/below part is the fixed mount which acts as a receiver for the other part, and the right/above part is affixed to the object (not depicted, but would be in the foreground) to be mounted. If it's not obvious, the intent of such a mount is to be fairly stable but still removable.
Another fairly common example of this kind is the button dashboard mount for a CB radio microphone (I'm not providing pictures of this due to copyright concerns, but a search for "cb button mount" in a search engine will show you examples).
I've considered "hammerhead mortise and tenon" because that seems to be mechanically similar, but I've never found that name used for this kind of mount.
Awarding the answer to @Akriss, French Cleat or its synonym split batten are a close enough match.