Define an abstract data type and provide an example.

An abstract data type (ADT) is a high-level description of a collection of data and the operations that can be performed on it.

In more detail, an abstract data type is a concept that encapsulates data structures and the set of operations that can be performed on them. It is 'abstract' because it provides a general idea of what the data type does, without specifying how it does it. The actual implementation is hidden from the user, which is a principle known as data abstraction. This allows the programmer to focus on what the data type does, rather than how it does it.

For example, consider a list ADT. The list ADT describes a collection of items, with operations such as 'add an item to the list', 'remove an item from the list', 'check if the list is empty', 'find the size of the list', and so on. However, it does not specify how these operations are implemented. The list could be implemented as an array, a linked list, or some other data structure. The user of the list ADT does not need to know or care about this implementation detail.

Another example is a stack ADT. A stack is a collection of elements with two main operations: push (add an element to the top of the stack) and pop (remove the top element from the stack). Again, the stack ADT does not specify how these operations are implemented. The stack could be implemented using an array, a linked list, or some other data structure.

In both these examples, the ADT provides a clear, simple interface to the user, hiding the complexity of the underlying implementation. This is a key principle of software engineering, known as abstraction. It allows complex systems to be built up from simpler components, each of which can be understood and tested independently.