During the past three decades, there has been an ongoing debate on the quality of health care. Defining quality is an important part of it. This paper offers a review of definitions and a conceptual analysis in order to understand and explain the differences between them. The analysis results in a semantic rule, expressing the meaning of quality as an optimal balance between possibilities realised and a framework of norms and values. This rule is postulated as a formal criterion of meaning, e.g. when (correctly) applied people understand each other. The rule suits the abstract nature of the term "quality." Quality doesn't exist as such. It is constructed in an interaction between people. This interaction is guided by rules in order to transfer information, e.g. communicate on quality. The rule improves our ability to discuss the debate on quality and to develop a theory grounding actions such as quality assurance or quality improvement.