Can anyone give me a definition of tone as defined in photography?

ok so this is perhaps embarassing to ask, but equally embarassing is that almost no dictionary I looked up could answer this question in an elaborate way. I am dong alot of photography these days, and though I think I know what tone is if it was given to me as a multiple choice question on an exam I cannot use it as part of my vocabulary because I am not sure what it refers to. Please help out and be specific with real examples.