A fluid is a substance that can move easily and change shape such as water (liquids), or air (gases), or plasmas. They can 'flow'- flow and fluids both come from the same English word roots (Latin: fluere "to flow") and are pronounced similarly. The physics of fluids is called fluid mechanics.
Fluids usually take on the shape of their containers, in contrast to solids which maintain their own shape.
If you cool a fluid down enough it eventually becomes solid.