SOFTWARE QUALITY ASSURANCE (SQA) is a set of activities for ensuring quality in software engineering processes that ultimately results, or at least gives confidence, in the quality of software products.
Definition by ISTQB
quality assurance: Part of quality management focused on providing confidence that quality requirements will be fulfilled.
Software Quality Assurance encompasses the entire software development life cycle and the goal is to ensure that the development and maintenance processes are continuously improved to produce products that meet specifications. Note that the scope of Quality is NOT limited to just Software Testing. For example, how well the requirements are stated and managed matters a lot!
Once the processes have been defined and implemented, Quality Assurance has responsibility of identifying weaknesses in the processes and correcting those weaknesses to continually improve the processes.
Capability Maturity Model Integration (CMMI) and ISO 9000 are some quality management systems that are widely used.