While in general engineers should define things, so that we can be clear about what we mean, there are plenty of things that are difficult to define that way. Love, happiness, satisfaction, for instance. You might argue "well those are emotions so they don't count" but you don't need to go far to find some more. What is the "perception of red"? What is the sensation of temperature (thermoception) or my sensation of my body in space (priprioception)? The sensations of these things are difficult to define--even if we have good explanations for how the physical world induces them--but they are experienced nearly universally by humans and we most people don't feel the need to define them to find them to be useful ideas.