ANSWER

Discount Usability Engineering was popularized by Jakob Nielsen as far back as 1989.  At the time called Usability Engineering at a Discount, it is still used today due to its basic underlying principles.  In this context, the term discount is used to symbolize a simplified and incomplete approach to usability engineering.  You can draw parallels to Pareto's 80-20 principle.  Nielsen argued that we need to keep usability evaluation methods quick, easy, and low cost in order for them to be readily understood by the average team member and to ensure that teams would actually complete usability evaluations on a regular basis. 

This isn’t to say that if given a choice he wouldn’t argue for carrying out more complete usability evaluations, but rather he realized that any action is better than no action. The majority of good usability principles could be realized by performing discount usability engineering.

So while Jakob Nielsen has spent decades of his career identifying and documenting thousands of very detailed usability principles he has also identified 10 heuristics that he feels incorporates the majority of those detailed rules.  By understanding and performing a heuristic evaluation using just those 10 heuristics you can achieve the majority of the benefit of a more detailed evaluation that takes into account hundreds of usability principles.  Similar to today’s Agile methods, this idea of “just enough” is what makes discount engineering processes so successful.