 What seems to be the most common practice today is, during the course of an investigation, to fix a list of important, characteristic properties of and phenomena associated with black holes required for one's purposes in the context of interest, and then to determine which of the known definitions imply the members of that list. If no known definition implies the list, one either attempts to construct a new definition that does (and is satisfactory in other ways), or else one concludes that there is an internal inconsistency in one's list, which may already be of great interest to learn. Examining the way the idea of black holes are used across physics—in astrophysics, cosmology, classical general relativity, semi-classical gravity, particle physics, various programs in quantum gravity, fluid mechanics, condensed matter, and analogue gravity—yields a list of potentially characteristic properties and phenomena some subset of which may plausibly be required or wanted in a characterization of a black hole in a given investigative context (Curiel 2019): 