The best simple definition I found is from Cracking the Coding Interview by Gayle Laakmann McDowell:

“An abstract data type is defined by its behavior (its operations).
The underlying implementation can vary.
You could implement a priority queue with an array or a min (or max) heap (or many other data structures).”

http://stackoverflow.com/questions/1692933/what-is-an-abstract-data-type-in-object-oriented-programming/1693029#1693029