Psycholinguistics, I'd argue, is a set of areas of study which overlaps as much into the core interest area of linguistics- language- as that of psychology- the (usually human) mind. And I don't just say this because I'm a psycholinguist in a linguistics department. Theoretical ideas in linguistics can be tested through psychological methods to see if it just looks pretty, or if it represents any of the truth about how real people use real language. Thus we keep the flightier kinds of linguist grounded, and pull the eyes of the more single-language-focused type towards the bigger picture. We're pretty much the superheroes of the Linguistics department. What we'd do in a psychology department I'm less sure. Make the tea, maybe?