We deliver a "keyword" account of the term life form as it has been used in natural philosophy and biology over the last two hundred years, beginning with its appearance in German as Lebensform. We argue that life form has, since its earliest enunciations, pointed to a space of possibility within which life might take shape, but that the way that space is imagined and theorized in biology has transformed substantially; life form originated as a term referring to idealized, aesthetic possibilities, then transformed to describe biogeographic and evolutionary potentialities, and today, in the age of synthetic biology and astrobiology, has come to signal conjectural and future possibilities.