An abstract data type (ADT) is a theoretical concept in computer science that defines a data structure purely by its behavior from the user's perspective. It specifies the operations that can be performed on the data and the types of values it can hold, without detailing how these operations are implemented. Common examples of ADTs include stacks, queues, and lists. ADTs help programmers focus on what the data does rather than how it is structured. This abstraction allows for easier code maintenance and flexibility, as the underlying implementation can change without affecting the code that uses the ADT. By using ADTs, developers can create more modular and reusable code.