What is the definition of Abstraction in relation to computer science?

Something that is an 'abstraction' is context free, meaning it applies outside of a 'real' example. Addition is an abstraction. One apple plus one apple is two apples. It could be apples, or oranges, or dogs, or cats, or ideas, or planets. A queue is abstraction. A queue could be a queue of people, places to travel, or nodes on a graph. Classes can be abstractions. They define the core tenets of what a class is, but might not be 'real' until subclassed and implemented (and even used). Layers can be abstractions. Asking for a primitive to be rendered on screen can be context free in discussion, but depending on which graphics rendering library you're using, can be several different things in 'reality'. So, so long as the 'reality' of the situation is hidden from you, and you can still move forward with your ideas, you're working with an abstraction.

It's a layer in which You perceive the problem. For a trivial example: When You want to boil some water You press a button on a kettle. It closes the electronical circuit and due to high resistivity of the metal board it produces heat which is absorbed by the water. But You don't need to know all of that to boil the water. You just need to know that when You click the button the water will evenually start to boil. Everything underneath is not important for You to solve the problem. That's Your layer of abstraction.