Being is an extremely broad concept
encompassing objective and subjective features of reality and existence. Anything that partakes in being is also called a “being”, though often this use is limited to entities that have subjectivity (as in the expression “human being”). So broad a notion has inevitably been elusive and controversial in the history of philosophy, beginning in western philosophy with attempts among the pre-Socratics to deploy it intelligibly.
As an example of efforts in recent times, Martin Heidegger (who himself drew on ancient Greek sources) adopted German terms like Dasein to articulate the topic. Several modern approaches build on such continental European exemplars as Heidegger, and apply metaphysical results to the understanding of human psychology and the human condition generally (notably in the Existentialist tradition).
Aristotle’s classificatory scheme had included the five predicables, or characteristics that might be predicated of a substance. One of these was the property, an essential universal true of the species, but not in the definition (in modern terms, some examples would be grammatical language, a property of man, or a spectral pattern characteristic of an element, both of which are defined in other ways). Pointing out that predicables are predicated univocally of substances; that is, they refer to “the same thing” found in each instance, St. Thomas argued that whatever can be said about being is not univocal, because all beings are unique, each actuated by a unique existence. It is the analogous possession of an existence that allows them to be identified as being; therefore, being is an analogous predication.
Whatever can be predicated of all things is universal-like but not universal, category-like but not a category. St. Thomas called them (perhaps not originally) the transcendentia, “transcendentals”, because they “climb above” the categories, just as being climbs above substance. Later academics also referred to them as “the properties of being.” The number is generally three or four.