Yale evolutionary biologist Stephen Stearns tackles a term biologists use constantly and define loosely: adaptation. Part of his Evolution and Medicine course, built around the 2015 Sinauer textbook Evolutionary Medicine, this short lecture lays out why calling a trait an adaptation is a claim that needs evidence, not just a label applied after the fact. Stearns walks through the criteria biologists use to test whether a trait actually evolved to serve a function, versus traits that are byproducts, historical leftovers, or the result of chance rather than selection. The lecture sets up a framework students will apply later in the course to human traits and diseases, where sloppy adaptationist reasoning has led medicine astray. It runs thirteen minutes and functions as one module in a numbered lecture sequence, pitched at students who already have the basics of natural selection.