canyoning and canyoneering are essentially the same thing, just with regional variation in what it's called. I propose that these tags be merged and a synonym created to avoid confusion. I've generally heard it referred to as "canyoneering", but that seems to be mostly only used in the US, with "canyoning" being more common elsewhere and having more questions tagged as such on the site. Therefore I suggest we merged "canyoneering" into "canyoning" and edit the tag wiki of "canyoning" to mention that it's also known as "canyoneering".
rappelling and abseiling mean the same thing, and the tag wiki for rappelling even says "also called abseiling". rappelling has more questions than abseiling (14 vs 5) and seems to be more commonly used in my experience, so I would suggest merging "abseiling" into "rappelling" and also creating a synonym.
Note: There has been much consternation about this post, and it seems to stem from my originally saying "merge request" and not "merge + synonym request". On other SE sites, posts tagged tag-merge-request are implied to be requesting a synonym be created at the same time. Apparently TGO users aren't familiar with that assumption, so I'm hereby clarifying that. I don't propose to remove wholesale all references to abseiling or canyoneering, I simply am proposing that we clean things up so it's easier for everyone to find it. If you disagree because you're afraid of the word "merge", please read the rest of this post very carefully before voting against this.
Edit: To everybody saying "no don't merge them, just make a synonym, merging is bad!", you don't understand how it works.
When you create a synonym, all future posts using the synonym tag are automatically and immediately retagged to the master tag. You can see an example of this with gear and equipment. Post a question with the gear tag and it will immediately be retagged to equipment as soon as you hit submit. This is how synonyms work. When the user is creating or editing a question, they can type in any of the synonyms and be presented with the relevant master tag, making it easy for somebody to type (for example) abseiling and see the rappelling tag complete with a little "also: abseiling" text below it.
All merging does is it retags any questions which had the synonymous tag before the synonym was created. This is just to ensure consistency and make it easier for people to search.
If you support making a synonym, there is literally no reason at all why you should not support merging the tags.
Further edit because apparently nobody believes me: Look, I'm a moderator on Super User, a Stack Exchange site 10x bigger than this one. I've merged tags more times than I can remember. I know how this works. Here is a screenshot of the freaking tag merge admin interface, so you can see exactly what I mean:
As you can see, there is a checkbox for "these two tags are synonyms, and mean the exact same thing". When you check that box and click "merge", any instance of the old tag is converted to the new tag, and a synonym is created mapping the old tag to the new one.
If you merge tag1 into tag2 and check the synonym box, then all questions with tag1 will be retagged to tag2, and any time somebody types
tag1 into the search box or the tag list on a question it will suggest