user-centered design
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Personas
Since Cooper invented personas, they've become the de facto interaction design tool across the world. Most folks know the basics of why: They embody salient user characteristics from first-hand user research in a medium that people are good at understanding, remembering, and discussing, i.e. people. I've been using personas as a design, teaching, and even writing tool for over a decade now and I believe strongly that all that's true. They do embody research. They are easy to remember and use. But I think there's another reason they work, a psychological reason, that's as important to understanding them: they get you to think about design problems in a fundamentally different way.