The AQL or Acceptable Quality Limit is the quality level that is the worst tolerable process average when a continuing series of lots is submitted for acceptance sampling.
Reference: ANSI/ASQ Z1.4-2003 Section 4.2
AQL is generally defined as "That level of quality acceptable to the consumer as a long term process average". In day to day terms, it means that the great majority of lots with quality equal or better than AQL will be accepted by the sampling scheme employed. They do not quantify the % acceptance of lots. For that you need to look at the OC Curve of the specific plan selected.
Also referred to as Assured Quality Level. The largest quantity of defectives in a certain sample size that can make the lot definitely acceptable; Customer will definitely prefer the zero defect products or services and will ultimately establish the acceptable level of quality. Competition however, will 'educate' the customer and establish the customer's values. There is only one ideal acceptable quality level - zero defects - all others are compromises based upon acceptable business, financial and safety levels.
For discussion, see these forum threads:
AQL - Assured Quality Level