flu·id. n. 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. adj. 1. Of, relating to, or characteristic of a fluid.2.. 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. adj. 1. Of, relating to, or characteristic of a fluid.