1. n. 
The act or means of controlling (usually lowering) the volume of filtrate that passes through a filter medium. Control of fluid loss for a mud is achieved by several means, one of which is by addition of fluid-loss-control materials to the mud system. Another is to change the mud chemistry to make the materials already present work better. Adding a clay deflocculant to freshwater mud typically improves fluid-loss control.
See: carboxymethyl starch, carboxymethylcellulose, filter medium, filtrate volume, fluid-loss-control material, gilsonite, gyp mud, high-pressure, high-temperature filtration test, low-pressure, low-temperature filtration test, plugging material, saltwater mud, spurt loss, static filtration