This tag is for requests to replace one tag with another existing tag. It also applies to requests that one tag should be merged into another, or simply abandoned.
This tag denotes a request for help to retag a set of questions. Note that the moderator capacity of merging tags is simply an extended application of retagging, and thus requesting that one tag be merged into another is simply a different flavor of retag request.
A completed retag request often but not always results in the creation of a tag synonym. You can add the tag synonym-request if you want the retag to be permanent.