How does one merge tags? Does it take a moderator? (The "multicolinearity" tag is a misspelling of "multicollinearity", for example, and should be removed permanently. Or should we allow misspelled synonyms?)
To answer you general question, you can create a synonym when you have 10K rep or are a mod (see the privileges page).
Regarding your particular question about multicollinearity, I agree with Srikant's suggestion:
- Misspellings should be fixed (as in this case).
- Context differences should be handled with a synonym, e.g. modelling and modeling.
I think the we should follow the following general recipe:
Misspellings like the one pointed out in the question should be fixed.
Misspellings because of a difference in usage across contexts (e.g., color vs colour) should not be fixed. Instead, we should use the synonym feature for these types of misspellings.