An ADT is:

The specification of a data type within some language, independent of an implementation
  • A concrete implementation of a class
  • An implementation for a data structure in a program
  • A collection of values
  • Another name for a data structure

A collection of values is a data type, not an ADT.

It can't be a data structure, since an ADT is more like the interface for a data structure.

An implementation for a data structure in a program is close, but not really it. An ADT is more abstract than an actual program.