No really well written definition anywhere for "Adventure Flight" but the interpretation can be had from the reason for creation of the Limited CofA and 262AM in the first place, which is to allow the general public to experience flight in a warbird. (if you want to get cynical it was to aviod having to administrate anyone that owned a warbird, but I like my view better).
I use the definition that it's any flight in a non standard category aircraft for the purpose of fun and fun alone.