I encountered this phrase on this site in a discussion of code metrics in Eclipse, specifically discussing the concept of "lack of cohesion":
Cohesion is an important concept in OO programming. It indicates whether a class represents a single abstraction or multiple abstractions. The idea is that if a class represents more than one abstraction, it should be refactored into more than one class, each of which represents a single abstraction.
What is a "single abstraction" in this context?
From Difference between Encapsulation and Abstraction, I got that abstraction generally is just showing necessary details to the user (through the use of interfaces and abstract classes). And here: What is abstraction?, I got again that abstraction is to hide implementation.
How would you apply these ideas to the single abstraction term used in that article?
In this particular context, defined in the site you quote (cohesion), "single abstraction" means one concept. So it is strongly related to Single Responsibility Principle - a class should deal with "one thing". If a class deals with more than one thing, it will often have many different methods and variables that don't necessarily belong together, thus it would have low cohesion.
This not a common usage of this word for this purpose, from my experience it is more likely to be used in reasoning about levels/layers of abstraction, like in the other SO questions you link. They relate to a different rule, namely the Single Layer of Abstraction Principle.