I think it’s code that’s been doused in Holy Water® and blessed at the Church of the Divine Pandorabot, by the High Priest Dr. Richard Wallace himself.
Seriously, though, “pure AIML” (if I remember the original context properly) doesn’t really have any special meaning at this point. It could refer to an AIML interpreter, such as Program AB, that only uses AIML files, and not some other means of storing the AIML categories. To contrast, Program O stores the AIML categories in a MySQL database. It even accesses them differently, when you get right down to it. In a very technical sense, Program O doesn’t use “pure AIML”, when examined from that perspective.
Alternatively, if you have a chatbot engine that merges AIML with some other technology (e.g. ChatScript or JAIL), then you have a chatbot that doesn’t use “pure AIML”. Of course, so far as I know there isn’t such an animal at this point in time, but there could be, one day.