java

An abstraction denotes the essential characteristics of an object that distinguish it from all other kinds of objects and thus provides crisply defined conceptual boundaries, relative to the perspective of the viewer.

from Object-oriented analysis & design with applications by Grady Booch

Read this book to know more about the OOA and OOD. You won't absorb the content in one sip. You need to read it again and again to digest it. You should change your thought process to do a real object-orientation.

Its not about coding in some OO language. Its about thinking in terms of real world objects and its responsibilities and collaboration.

HTH

Hmmm...
IMO, Abstraction is a process of hiding the details of an entity by just providing the vague or overview of the entity...
This definition means the same as that of the above comment.
In the above eg, we are not informed about the type, size or any such property of the book. All these details are associated with the book.
Hope you got it. :)

I hope you are trying to find out what is Abstraction and not Abstract Class.

I will give an example for Abstraction but i will leave it to you to write down the definition with whatever you have undestood.

Suppose i say Book then there some basic features which we think about.
e.g: No of pages, Size, language etc.

Then what happens if i tell you say Head First Series Java book, you may get same information as above, also you will get some additional information you will also come to know that its a java book.

Now if you can identify the diff between when i say Book and Java Book.

Here i call Book as an Abstract Entity which gives generailzed information not full details.

I hope this helps.

/H