1 liquid, solution, liquor, ichor; gas, vapour:
It might surprise some to learn that physicists consider gases to be fluids.
2 liquid, flowing, running, runny, watery, aqueous:
Once the metal has become fluid, it is poured into the moulds.
3 changeable, mutable, flexible, adjustable, variable, pliant, unformed, formless, unfixed, unstatic or non-static, plastic, protean, mercurial, mobile, unstable, shifting, uncertain, indefinite, unsettled:
The situation is fluid and the decision could go either way.