Is the Open Domain Question Answering really "open"?
Although the popular description of the Open-Domain QA centers on the fact that the questions are not limited by a specific domain, this is not totally correct unless the term domain is used in the philosophical sense of a sphere of knowledge, influence, or activity, which is listed as 'sense #4' in the Merriam-Webster's dictionnary.
However, the term domain may also have a mathematically sounding meaning of a ...set of elements to which a mathematical or logical variable is limited; specifically : the set on which a function is defined. See 'sense #5' of the same dictionary. In this case, it can be said to be equivalent to the CS term data.
As evaluation conferences like TREC teach us, there is always a fixed amount of input data for a QA system. Which means that every QA system participating in TREC is to be called a closed-domain system if the term domain is used in its 5th sense.