1. n. 
An assembled length of steel pipe configured to suit a specific wellbore. The sections of pipe are connected and lowered into a wellbore, then cemented in place. The pipe joints are typically approximately 40 ft [12 m] in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings. Long casing strings may require higher strength materials on the upper portion of the string to withstand the string load. Lower portions of the string may be assembled with casing of a greater wall thickness to withstand the extreme pressures likely at depth. Casing is run to protect or isolate formations adjacent to the wellbore.
The following are the most common reasons for running casing in a well:
See: bell nipple, blowout preventer, BOP, box, casing, casing shoe, conductor pipe, coupling, displacement, dogleg, float joint, intermediate casing string, joint, liner, mill, pin, reciprocate, surface casing