A data type that has its behaviour defined, but lacks an explicit implementation. Abbreviated as ADT.
As an ADT only describe the logic, it is language agnostic, meaning that it can be implemented in most programming languages. Once it is implemented, it is considered to be a data structure, no longer being an ADT.
Note: not all coding languages implement all ADTs and they may implement ADTs differently with different functionality, thus it is important to distinguish the usage of ADTs in pseudocode.
Motivation
To hold data and information more effectively, we need to model the general form that it has, and what we expect to do with it. Hence, we use ADTs to abstract away the implementation details, presenting a simpler interface that reduces complexity, and facilitating greater functionality and readability of algorithms.1
Signature
Each data type contains a signature, which provides information about the possible operations and outputs.