Ghost Speak or Ghostie Speak is a method people on NaNo often used to write information that some people might want to see and others might want to avoid, such as triggering material or the answers to riddles. Sometimes people just write in it for fun.
A user encoding a message this way will hide the text within the web formatting code of a comment by putting HTML brackets (< >) around the partially hidden content. As a result, someone viewing the comment on a thread cannot see the ghostie speak, but if someone quotes the comment then they will see the HTML code, and therefore the ghostie speak, of the other comment.
In 2013, the mods forbid anyone to use ghostie speak, as it was too hard for the mods to track. It is now against the rules.