I keep reading authors refer to "c number equations" in different texts on quantum optics. Perhaps I am missing something very obvious, but what exactly does this mean? Does it mean complex-valued equations? Does it have to do with the speed of light? Does c describe the number of equations being evaluated? Here are a couple examples: 1st paragraph of page 190 Section 6.2 in this text I've just seen the term used in different contexts yet never had its meaning clarified. Maybe it's so blatantly obvious that it does not require explanation. But nevertheless, I am looking for one.