Definition of "fluid" []

• 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. (noun)
• Of, relating to, or characteristic of a fluid. (adjective)
• Readily reshaped; pliable. (adjective)
• Smooth and flowing; graceful: the fluid motion of a cat. (adjective)
• Changing or tending to change; variable: a fluid situation fraught with uncertainty. (adjective)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Around each joint has grown up a strong sheath of tough, fibrous tissue to hold the bones together; and, inside this, between the heads of the bones, is a very delicate little bag, or pouch, containing a few drops of smooth, slippery fluid (_synovial fluid_) to lubricate the movements of the joint."
• "As the layers become continuous with each other at different points, the arachnoid, like the pericardium, forms a shut sac, and, like other serous membranes, it secretes a fluid, known as the _arachnoid fluid_."
• "Moving over matter which has the qualities that we denote by the term fluid, the swayings which the air produces are of a peculiar sort, though they much resemble those of the fiddle string."