'Cost of quality (COQ) is a technique for identifying the costs of meeting the requirements, including any necessary rework. This technique is useful for the PMI processes to plan quality management and estimate costs.
It is often useful to examine the components of the cost of quality: the cost of conformance and of nonconformance. The conformance costs are those that avoid failure and get the deliverables right. Some examples are training, inspections, "time to do it right". The nonconformance costs are the ones needed to fix failures or mistakes. Examples of nonconformance costs are unusable deliverables, damage to reputation, and warranty or recall costs.
Cost of quality is defined in PMBOK section 184.108.40.206 (5th edition).