S.R. Ranganathan is credited with the introduction of the term facet in the field of knowledge organization, towards the middle of the Twentieth century. Facets have traditionally been used to organize document collections and to express complex subjects. In the digital world, they act as filters to facilitate navigation and improve retrieval. But the popularity of the term does not mean that a definitive characterization of the concept has been established. Indeed, several conceptualizations of the facet co-exist. This article provides an overview of formal and informal definitions found in the literature of knowledge organization, followed by a discussion of four common conceptualizations of the facet: process vs. product, nature vs. function, object vs. subject, organization vs. navigation.