Quality

ISO 9000:

degree to which a set of inherent characteristics fulfils requirements

ISO 25000:

degree to which the system satisfies the stated and implied needs of its various stakeholders, and thus provides value

Essentially, quality is just a match between what was desired and what is produced.

Quality Management Definitinon

Quality Assurance

ISO 9000:

part of quality management (3.2.8) focused on providing confidence that quality requirements will be fulfilled

ISO 25000:

Testing