An abstraction is a simplified view of an object in the user's own vocabulary.
In OO ,an abstraction is the simplest interface to an object that provides all the features and services the intended users expect.
An abstraction tells users everything they need to know about an object but nothing else. It is the well-defined, unambiguously specified interface. For example, on a vending machine, the abstraction is formed by the buttons and their meanings; users don't have to know about levers, internal counters, or other parts that are needed for the machine to operate. Furthermore the vending machine's price list implies a legally binding promise to users: if users put in the right amount of money, the machine promises to dispense the desired item.
The key to a good abstraction is deep knowledge of the problem domain. A good abstraction allows users to use an object in a relatively safe and predictable manner. It reduces the learning curve by providing a simple interface described in terms of the user's own vocabulary.
A good abstraction separates specification from implementation. It doesn't expose too much nor does it hide features that users need to know about. If an abstraction is good, users aren't tempted to peek at the object's implementation. The net result is simpler, more stable user code.