Well, you would not believe how many people cannot give you a definition, even if they use computers all day long. Find out yourself and ask some people what a computer is. You may be surprised how weird answers will you get.
Search Wikipedia or google for it to see if you can find a workable definition that makes sense.
I couldn’t, so I have invented one:
Computer is an electronic device, a machine, that is used for:
of data. By data, I mean anything a person can be aware of, that can be put in the computer.
I’m not saying that this is the only right definition of computer, however, it always worked me when teaching people it even works very well for developers to understand MVC (Model-View-Controller) architecture better.