Nowadays when we talk about "perennial plants," or simply "perennials" (perennial can be a noun, too), we mean plants that die back seasonally but produce new growth in the spring. But originally perennial was equivalent to evergreen, used for plants that remain with us all year. We took this "throughout the year" sense straight from the Romans, whose Latin perennis combined per- ("throughout") with a form of annus ("year"). The poet Ovid, writing around the beginning of the first millennium, used the Latin word to refer to a "perennial spring" (a water source), and the scholar Pliny used it of birds that don't migrate. Our perennial retains these same uses, for streams and occasionally for birds, but it has long had extended meanings, too.