A continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container; a liquid or gas. Of, relating to, or characteristic of a fluid. Readily reshaped; pliable. Smooth and flowing; graceful: the fluid motion of a cat. Changing or tending to change; variable: a fluid situation fraught with uncertainty. Characterized by or allowing social mobility: a fluid society. Convertible into cash: fluid assets.