The early French and Latin roots of the verb mean something closer to "to limit" or "to create a boundary," but this is really what we do when we define something: we limit its meaning to something specific. So when you're thinking about the word define, remember that you're looking to create a boundary of meaning around it, whether you're specifying the meaning of a word or a larger concept. To define a word such as astronomy is to define both the word and the realm of science that it's concerned with.