Definition of Critical to Quality (CTQ):
The internal critical quality parameters that relate to the wants and needs of the customer. They are not the same as CTCs (critical to customer), and the two are often confused.
CTCs are what is important to the customer; CTQs are what’s important to the quality of the process or service to ensure the things that are important to the customer.
A quality function deployment (QFD) or CTQ tree relates the CTQs to the CTCs. For instance, car door sound when closing might be a CTC, while the dimensional tolerances and cushioning needed to produce those conditions are CTQs for the auto maker.