And now it's moved to the inaccuracies of the plumbing personelI really hope I don't give that impression, I enjoy a good read.
Yes it is a tank, assuming we are referring to the tanks installed in domestic properties in lofts to provide a header to the vented heating system and the domestic hot water and also to provide the flushing facility for toilets.
Yes they are all water tanks, indeed as the hot water cylinder is too.
The fact that we have abbreviated 'cistern tank' to 'cistern' doesn't stop it being a tank. It isn't restricted to water as it can equally be any fluid such as petrol.