Abstraction means forming simplified conceptual models of problems that enable general or reusable solutions. For example, functions are an abstraction with very general applicability to solving the problem of structuring and reasoning about code. Abstractions with high generality are called rich abstractions.

Problems in complex domains like computing are solved using many stacked layers of abstraction. Program execution can broadly be decomposed into a computational model that is implemented in a machine model using an execution model, where the machine model is at the bottom of the stack. The layered architecture is relevant to understanding that contradictory or seemingly exclusive concepts can co-exist at different layers of abstraction, like concurrent execution on a serial machine. When the contradictions between an abstraction and the underlying model are exposed and the abstracted-away details can't be ignored, it's called a leak in the abstraction.

Dealing with leaking abstractions is a key aspect of concurrency modeling and programming in general, so this glossary emphasizes the possible contradictions or branching paths in the design space of concurrency models by grouping the abstractions as dichotomous pairs of terms. The topics are organized in an ascending order of abstraction level instead of alphabetically.