  This paper presents a taxonomy of explainability in Human-Agent Systems. We
consider fundamental questions about the Why, Who, What, When and How of
explainability. First, we define explainability, and its relationship to the
related terms of interpretability, transparency, explicitness, and
faithfulness. These definitions allow us to answer why explainability is needed
in the system, whom it is geared to and what explanations can be generated to
meet this need. We then consider when the user should be presented with this
information. Last, we consider how objective and subjective measures can be
used to evaluate the entire system. This last question is the most encompassing
as it will need to evaluate all other issues regarding explainability.
