Lower Specification Limit (LSL)

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