1. n. 
A type of lost circulation material (LCM) that is long, slender and flexible and occurs in various sizes and lengths of fiber. Fiber LCM is added to mud and placed downhole to help retard mud loss into fractures or highly permeable zones. Ideally, fiber LCM should be insoluble and inert to the mud system in which it is used. Examples are cedar bark, shredded cane stalks, mineral fiber and hair. Often, granular, flake and fiber LCMs are mixed together into an LCM pill and pumped into the well next to the zone of fluid loss to seal the formation that is taking mud from the system.