From the Latin, the "negative way." It is a method of investigative teaching in which one does not define the correct way, but constantly rejects obvious mistakes. One can describe what it isn't, but not what it is. It can be a frustrating, even painful process as the student zigzags forwards from rejection to rejection. Eventually students (if they survive!) learn to be responsible for finding their own ways into the unknown. The method develops strong, curious, self-sufficient artists.
The term comes originally from the Roman Catholic tradition, the idea being that no mortal can say what God is, only what God isn't.