Background: The topic "quality" has gained importance during recent years due to increasing interest of the society, insurances, governmental institutions and, last not least, those who are involved in health care themselves. Definitions: The terms and notions, which mostly are adopted from the industry, are not at all self-explaining. Clarifications not only serve to avoid misinterpretations. The efficiency and power of the underlying concepts can not be recognized without introduction into the proper terminology.