(of a person) able to be contacted in order to provide a professional service if necessary, but not formally on duty. You’re kind of working, but you’re also kind of off of work when you’re on-call. You may not be at your workplace now, but you could be headed there at the drop of a hat. You’ll be able to attend social obligations. Which is great, but you’re not able to do anything that would hinder you from leaving immediately. (Like drinking a beer or two)