Basically if we talk about the software quality there is no clear answer to it. The answer varies from person to person. So following any one of the person,
According to the Pressmen, they defined the software quality as the confirmance to the explicitly started functional and the performance requirements. It is the explicitly documented development standards and implicit characteristics that are expected from the professionally developed software.
Therefore, this answer serves to highlight the 3 important points:
Software requirements and specifications are the building blocks which measures the quality suggesting that if there is lack of confirmances to requirements, there is lack of software quality.
Specified standards defines the set of criteria for the development that helps guiding the manner in which the software is engineered. So until and unless the criteria is followed, the software quality is surely achieved.
There is the set of the implicit requirements which is often unmentioned. If the software meets the explicit requirements but fails to meet the implicit requirements, software quality degrades.
In general, one can understand that the software quality is a complex mix of the factors that varies from person to person and from system to system.