CTM is defined by ISO 13250-6, that is, part 6 of the Topic Maps standard. It's a textual notation to describe topic maps.
CTM is currently out for FCD ballot.
More information:
Meeting notes:
The current editors are Lars Heuer, Gabriel Hopmans, and Dmitry Bogachev. Feedback can be sent to them or to the topicmapmail mailing list.
Other material: