What is Ceremonial High Magick (CHM)? I don't have truly clear definitional differences between "High" magick and "Low" magick and "Ceremonial" magick, but fundamentally thinking along these lines seems to work for me: High magick is essentially any magick designed to move the magician closer to the Knowledge and Conversation of the Holy Guardian Angel, or in other words, magick towards a spiritual end. Low magick, it follows, would be magick designed to have a direct, physical impact on the world. This definition doesn't sit well with all people, so don't take it as gospel. Other ways I've seen "High" and "Low" magick differentiated involves the way it is done; i.e., low magick will include things like lighting candles, saying some words from a book and things happen; high magick will involve formal rituals which interface with the Archangels and the God-names, etc. Somewhere in each of these two sets I find a little bit of meaning which resonates, but which defies my ability to put it to words.
I've seen "Ceremonial Magick" used interchangably with "High Magick", and have also seen it defined as rituals in groups vs. solitary ritual. Not sure I buy into the latter definition, but this is all just semantics anyway.