To see all the tags then synonyms go to:
"Every question on Stack Overflow, or any other Stack Exchange site, must be tagged with at least one tag. Tags are simple keywords or labels that help categorize your question with other, similar questions"
"We added tag merging about a year ago, but it was purely a moderator function. We’ve also slowly but surely been increasing the reputation required to create a new tag from 250 reputation, to 500 reputation, all the way to the current setting of 1500 reputation. We’ve also started automatically culling single-use tags that are more than 6 months old."
Moderators do have the option to accept or decline the changes or merge tags but there is some clean-up from the Stack Exchange employees.