U chart: Count per unit chart.
Unilateral tolerance/one-sided specification limit: A form of tolerance equal to the minimum or maximum performance requirement beyond which a measured performance parameter is considered to have failed. There is an upper bound or a lower bound, but not both. The characteristic must either equal, or fall above this limit or below it, as specified, in order to be acceptable. On one side there is conformance, on the other, non-conformance. Also called "limit," "specification limit." May be an upper specification limit or lower specification limit. Contrast with double specification limit. Given by: nominal plus tolerance or nominal minus tolerance. (Quality Council of Indiana)
Unit: An object on which a measurement or observation can be made. Note: Commonly used in the sense of a "unit of product,"the entity of product inspected in order to determine whether it is defective or nondefective.
Upper control limit (UCL): Control limit for points above the central line in a control chart.