Here is what Wikipedia says at the time of this writing:
A software bug is an error, flaw, failure, or fault in a computer program or system that causes it to produce an incorrect or unexpected result or to behave in unintended ways.
I think that's incomplete. The definition entirely excludes "non-behavioral" defects related to, for example, maintainability and reusability.
As you know, every piece of software has functional and non-functional requirements. Functional requirements tell us what the software has to do, and non-functional requirements document how it has to do it. For example, here is a functional requirement:
If our software doesn't generate a PDF report and crashes instead, that's a functional bug. If instead of a PDF report, it generates an empty page or a plain text document, that's a functional bug. If there is no "generate PDF report" button at all and the user simply can't start the PDF generation process, that's a functional bug.
Here is an example of a non-functional requirement:
If our software generates a perfectly correct PDF report but it takes a minute, that's a non-functional bug.
So far so good, since the bug definition given by Wikipedia perfectly covers both of them—if they happen, they will cause our software "to produce an incorrect or unexpected result or to behave in an unintended way." The emphasis here is on the words "produce" and "behave." They presume the software is doing something and we're observing its behavior.
However, that's not all of it.
What about maintainability? I may have this kind of non-functional requirement:
It's a rather vague requirement, but you get the idea.
Maintainability and reusability are very critical non-functional components of any modern software program, especially taking into account a very high cost of labor in the market. Very often, it's more important to make sure the software is maintainable than fast. If it's maintainable and slow, we can find new programmers to improve the code. If it is fast but unmaintainable, we won't be able to do anything with it later and will have to rewrite it from scratch if some new feature is required. Read more about this in Are You a Hacker or a Designer?.
The definition of a software bug given by Wikipedia doesn't cover maintainability and reusability flaws at all. That makes for a common source of confusion—an inconsistent code style is not a bug (see the discussion under this post).
That is wrong.
An inconsistent code style is a software bug, as is incomplete documentation, lack of documentation, code that's too complex, the lack of a coding style guide, etc.
I would rewrite the software bug definition paragraph in Wikipedia like this:
A software bug is an error, flaw, failure, or fault in a computer program or system that causes it to violate at least one of its functional or non-functional requirements.
This definition looks more accurate to me.