I've always called "tag" the id that optionally follows struct/union/enum. Is it the wrong word? Some specs call it "name", but "unnamed union" sounds dangerously similar to "anonymous union", which is a different concept, namely (no pun!) an unnamed member of an outer struct or union whose submembers can be accessed as if they belong in the outer one. E.g.

  struct {
    struct {
      int m;
    }; // no name: anonymous
  struct s;
  s.m = 1;