Fluid is a generic term for a material which will flow, i.e. anything except a solid.
Actually -- even solids can behave like fluids. A glacier is a solid that flows -- as a solid. Veeeeeery slowly.
Now there's some ice you don't want in contact with your pitot tube, or wings, or anything else ... ;D
(Complete aside -- I actually got to go and see, and walk on, some pretty amazing glaciers in Alaska this summer. Guess where a lot of my fs9 flights have been since? :-).
My question is -- how does one set icing conditions in fs9? Go to a cold part of the world and fly in winter? Is there a way to control temps and stuff in the weather menu? Thanks y'all!