fluid /flo̅o̅″id/ [L, fluere, to flow] , 1. a substance, such as a liquid or gas, that is able to flow and to adjust its shape to that of a container because it is composed of molecules that are able to change positions with respect to each other without separating from the total mass. 2. a body fluid, either intracellular or extracellular, involved in the transport of electrolytes and other vital chemicals to, through, and from tissue cells. See also blood, cerebrospinal fluid, lymph.
