1. n. [Well Completions]

An isolation tool set in the casing or liner that enables treatments to be applied to a lower interval while providing isolation from the annulus above. Cement retainers are typically used in cement squeeze or similar remedial treatments. A specially profiled probe, known as a stinger, is attached to the bottom of the tubing string to engage in the retainer during operation. When the stinger is removed, the valve assembly isolates the wellbore below the cement retainer.