Upper Specification Limit (USL)

A statistical reference for the upper level target of a continuous data point defined by the customer such that anything above it is considered a defect or not meeting the customer's needs (a.k.a. voice of customer or VOC). It essentially represents the maximum measurable point of tolerance for the customer. This is contrasted with the lower specification limit (LSL).
Related StatStuff Video: 
   -Section 5-Measure Phase, Lesson #21 - Defining the VOC and Defects