U chart (count-per-unit chart)

A type of control chart used to monitor the number of defects per unit in a process when the sample size varies.
See also: Control Chart, C Chart

USL (Upper Specification Limit)

The highest value within acceptable limits for a process or product characteristic, as defined by customer or engineering requirements.
See also: Specification Limits, LSL (Lower Specification Limit)

Unit

A single item, product, or entity that is measured, produced, or tested in a process.
See also: Sample, Lot

Unit test

A test that verifies the functionality of a specific component or section of a process, product, or software.
See also: Verification, Validation

Upper control limit (UCL)

The highest value on a control chart that a process statistic can reach before signaling that the process may be out of control due to special cause variation.
See also: Control Chart, Lower Control Limit (LCL)

Upper specification limit (USL)

See: USL (Upper Specification Limit)