A matrioshka brain is a hypothetical megastructure proposed by Robert Bradbury, based on the Dyson sphere, of immense computational capacity. It is an example of a Class B stellar engine, employing the entire energy output of a star to drive computer systems. This concept derives its name from Russian Matrioshka dolls
I personally met Robert Bradbury at several of the Foresight conferences. He was a pleasant and interesting person to have a conversation.