1. n. [Well Completions, Well Workover and Intervention]

A sleeve with a flanged top connection upon which a frac tree is installed. The sleeve is inserted into a wellhead to isolate its low-pressure connections from the higher fracturing pressures. The fracturing mandrel enables the use of low-pressure wellheads that could not normally withstand the higher pressures necessary for hydraulic fracturing.

See: fracturing sleeve,  mandrel