I think that the tags [tag:taylor-expansion] and [tag:taylor-series] should be synonyms. (The latter is probably relatively new. It is still shown in the [list of the new tags](http://math.stackexchange.com/tags?tab=new) and it contains only 17 questions at the moment.)

EDIT: Users with sufficient reputation in the [tag:taylor-expansion] tag can vote for (or against) this synonym [here](http://math.stackexchange.com/tags/taylor-expansion/synonyms).