Stats on the two tags (as of time of writing):
parenthesis: 33 questions total, 2 unanswered, no tag wiki written
parentheses: 30 questions total, 3 unanswered, well-written tag wiki
I came across this when retagging one of the questions on TeX.SE involving parentheses. Considering how these two terms are just singular/plural forms of each other, I suggest we make them synonyms.