I'm unable to understand the difference between 液体 and 流体. Both sound like "fluid" to me, while the latter sounds like "flowing fluid" (if that makes sense). I had thought there was only 気体、液体、and 固体. What is this 流体 and how can it be explained?
Is it not the difference between fluid and liquid? The latter can be very viscous or "fluid"/free flowing?
Defn of fluid: a substance that has no fixed shape and yields easily to external pressure; a gas or (esp.) a liquid
It is a bit clearer if you look at the origins of the words.
Fluid (as an adjective): from French fluide or Latin fluidus, from fluere ‘to flow.’ Liquid: from Latin liquidus, from liquere ‘be liquid.’