StackExchange sites use tags to help users sort, search, group and filter the large volume of questions and answers.
Sometimes there are cases where a tag has a common misspelling, or a meaning similar to another more common tag, or describes a (non-useful) subset of the things described by another tag. Because SE wants tags to be as useful as possible, and used as uniformly as possible, it provides a mechanism to automatically turn the less useful of a pair of tags into the more useful of that pair. StackExchange calls these tag pairs tag synonyms and provides a user interface where high-rep users can propose new synonyms for a tag. Here's an example of the current user interface (from the Computer Science StackExchange).
Personally, I can't tell whether it has been proposed that notation be used instead of terminology or that terminology be used instead of notation. And apparently no-one else can figure it out either, because this user interface is used wrong-way-around about as often as it is used as intended. Part of the problem is with the use of the term synonym. A synonym is
a word or phrase that means exactly or nearly the same as another word or phrase in the same language [Google].
That is, synonymity is an equivalence relation. In particular synonymity is symmetric. Given a pair of tags a and b, "a is a synonym for b" implies "b is a synonym for a." But that's not how StackExchange tag "synonyms" work. The system is supposed to either automatically convert b into a (always) or automatically convert b into a (always). That is, it's a hierarchy.
What term(s) should StackExchange be using here?
(Just for the record there is apparently another place, in the StackExchange moderator tools, where they use non-symmetric terminology, but it is still not clear which way the replacement is intended. See https://meta.stackexchange.com/questions/142891/fixing-the-sucktastic-merge-ui-for-tags.)