There is no one definition of quality improvement. 

The Paul Batalden and Frank Davidoff quote below is widely accepted as one of the best-

“The combined efforts of everyone – healthcare professionals, patients and their families, researchers, commissioners, educators – to make changes that will lead to better patient outcomes, better system performance, and better professional development.” – Paul Batalden and Frank Davidoff

Improving quality is about making health care safe, effective, patient-centred, timely, efficient and equitable. In the history of the NHS, there has never been a greater focus on improving the quality of health services.

https://www.health.org.uk/publications/quality-improvement-made-simple