Have a look at Create Tag Synonyms privilege
Users with more than 2500 reputation and a total answer score of 5 or more on the tag, can suggest tag synonyms. Users with a total answer score (total upvotes minus total downvotes) of 5 or more on the tag, can vote for tag synonyms.
To suggest synonym go to the tagged question page (i.e. click on any tag) > click on
You will be redirected to the synonym page:
Here you can suggest synonyms if you have enough privilege and score on that tag.