Usually, I go with the definition of ‘We are the synthesis of three fields of expertise: Mechanical Engineering, Electrical Engineering and Information Technology. We can act as a link between those three, possessing a wide and fairly deep kind of knowledge at the same time, or we can specialize in one single field of research (various things, like robotics, control systems, electrical motors, and many more) and attain as narrow and deep an expertise as any of the engineers who already started out in it.’

The problem still stands, I have not found a concise, fit-to-the-meanest-understanding definition of Mechatronics yet, and I do not think that there will soon be one.