There are quite a few questions on measuring and proving software quality here, however, to this point, there was no formal definition of software quality itself on Stackoverflow:
Please give formal, all encompassing definition of software product quality.
Since the question is fairly generic I am happy to keep accepting a better definition each time it is submitted. It is OK to copy the gist of previously submitted answer as long as you can add something valuable to it.
A quality software product must
EDIT: (Things like documentation, portability, efficiency etc. are desirable bonus values, but not required to make it "quality", but to make it comparable, IMHO)
Here's what Wikipedia has to say about software quality.
The article lists several definitions and views on software quality. Closest it comes to Software Product Quality is
Conformance to requirements or program specification; related to Reliability.
Reliability is "the probability of failure-free operation of a computer program in a specified environment for a specified time."
Along with product quality it lists:
A quality factor is defined by "a non-functional requirement for a software program which is not called up by the customer's contract, but nevertheless is a desirable requirement which enhances the quality of the software program." Here's the list:
Edit: Clarified what quality factors are supposed to mean.
I once heard that quality is like Oxygen. When it's there, you take it for granted. When it's not there, you'd notice it and you complain. So it may be easier to define quality from the stand point of what "bad quality" software is. For example, an insecure online bank website would be a "low quality" product.
Software product quality is the inverse of the rate at which software product defects are noticed.
This does not mean that if no bugs are reported you have good quality, because you might have made it too difficult to report problems. Also, if a problem exists, but isn't actively noticed, it does not affect quality much, but if it is noticed constantly, it is a major quality influence. A button that is badly misaligned seriously degrades quality, even if it does not affect functionality.
Since it is often the user who notices a defect, it also doesn't matter whether something is a bug or a feature according to the developer. If the user perceives something to be a defect, it is a defect, whether in user training, user documentation, user interface or actual functionality.
There is no one answer to this and many accepted answers directly contradict one another. Some popular ones include;
"Quality is value to some person or persons" - Jerry Weinberg, in Quality Software and Testing, April 2006
"Quality is conformance to standards" - Philip Crosby
"Quality is the ability to stay in business" - Demming
We had a good discussion on this over at SQAforums here. The broad consensus is that quality is largely subjective. Personally, I go with Jerry Weinbergs take on it.
A formal definition of software quality is in fact available in the form of ISO-9126 as referenced by the wikipedia article: http://en.wikipedia.org/wiki/ISO_9126
http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=22749 defines the quality model
Our programs are FUN to use.
Our instructions are CLEAR and complete.
Product quality is a multitude of all its functional and non-functional characteristics. The measure of quality varies depending on the context of the actual product use.
A software product can be characterised by:
Functional attributes (or what work the product performs)
Non-functional attributes (product properties other than work it performs)
a) Look and Feel
b) Usability and Humanity
c) Performance
d) Operational
e) Maintainability and Support
f) Security
g) Cultural and Political
h) Legal
Classification of functional and non-functional properties as suggested by James and Suzanne Robertson http://www.volere.co.uk
My favourite quote regarding quality is:
Quality is never an accident; It is always the result of high intention, sincere effect, intelligent direction and skillful execution; it represents the wise choice of many alternatives.
If I was to slap a metric on quality, I think the best external metric is "how much are people willing to pay for it". No one wants to pay for lousy product.