I usually consider a WHAM to be any moment in a story that could cause the reader a considerable amount of angst and potential heart-trouble to read. This could be any number of things, from a character death to your written tensely near-death Clark or Lois is in peril situation.
I don't care one way or the other if there is a warning about these things or not, but I generally include the kind of mood a reader can expect to find in my author's notes when I've written something.
