This chapter centers on a definition of Documentary by the renowned media historian Erik Barnouw (1908–2001). In this chapter I take pains to demonstrate how and why the terms by which Barnouw researched, produced, and wrote his 1974 work Documentary: A History of the Nonfiction Film, serve as a fundamental gauge for our comprehension of its philo-historical and documentarological value—in fact, constituting Barnouw’s novel and idiosyncratic definiens. Barnouw is the third honorary member in a triumvirate of film historians who revolutionized the form, style, scope, and conception of narrativizing the history of the documentary film in the early 1970s. The chapter argues the following: while Barnouw’s view that Documentary can be defined in terms of its subjective agent—function-documentarian—had already been conceived and presented earlier in the decade, though in different terms, by Soviets and British alike, I contend that the Barnouwian idiosyncratic effort to define Documentary in terms of fourteen categories of the function-documentarian broke ground for the academic understanding of not only what Documentary is or isn’t but, more interestingly, about the infinitely complex relations between the function-documentarian and documentariness.