I’m asking people this question. Seeking peoples definitions in simple terms. Here I will compile the answers. If this question was “what is A cinema” we would get answers that only relate to he institution of a building and its use and this would weight the enquiry too much in one (predicatable) direction.
James Reed “Film based projection” (he was biased, I’d just summarised what I was up to).
See this for further practical reasons for thinking that film based projection is the ontological basis of ‘Cinema’.