A tongue-in-cheek reply: well I may not be able to define it, but I know it when I see it.
If one has some sort of machine, that can be controlled by information that can be recorded in some time-persistent medium, where there has to be some kind of rules expressing what can be recorded, that set of rules could be called a programming language.
If the machine being controlled happens to be universal (in the Turing sense) then it's what we would typically call a computer, but simpler machines can also be programmed. I'm thinking of the Jacquard loom, or various controllable machine tools.
