1. n. 
The collective term for substances added to drilling fluids when drilling fluids are being lost to the formations downhole. Commonly used lost-circulation materials include are fibrous (cedar bark, shredded cane stalks, mineral fiber and hair), flaky (mica flakes and pieces of plastic or cellophane sheeting) or granular (ground and sized limestone or marble, wood, nut hulls, Formica, corncobs and cotton hulls). Laymen have likened lost-circulation materials to the "fix-a-flat" materials for repair of automobile tires.
Alternate Form: LCM