I went ahead and used the power of the mystical diamond to swap the synonym with the canonical name. Oddly, there are 76 questions tagged "xp" and only one tagged "extreme programming", but synonyms allow for the flexibility of users choosing the tag they prefer.
Up, up, and away to do more good deeds! :)
I went ahead and merged the tags and created wbs as the synonym of work-breakdown-structure.
I'm not an expert on tag synonyms, but it looks like anyone with 1250 reputation can propose or vote to approve a synonym. The privileges page on suggesting tag synonyms has more details.
I believe this might answer your question:
Users with more than 1250 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. Suggestions will be automatically approved
when they ...
I don't see a difference in the definitions, so I agree that they should be made synonyms. I also agree that estimating should point back to estimation as the canonical name for the activity. Additionally, I'd suggest adding estimates as another synonym.
Apparently, I created the work-in-progress tag, but I don't have enough answers in the tag category to create sensible synonyms yet. This will require moderator intervention in the future, if enough related tags needs to be rolled together (see answer by @jmort253).
Question is Mis-Tagged
status-completed is actually wrong; this should have been ...