system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is an abstract data type (ADT)?

Is a ADT just: int x = 5;

and why is a Linked List not a ADT?
gpt
An abstract datatype is like a machine that has a control panel.  The user(programmer) can put an item in, take items out, ask questions about the number of items, things like that.  But the user(programmer) has no idea, and shouldn't know, how the machine is keeping track of the items.  

For a real example there is a list type in c#.  It doesn't matter if it is stores items as a linked list, an array, or whatever.  All you(the programmer) care about is that it is a list.  

An ADT is just like an interface but non-language specific.  So no matter what language you're working in if you are writing a List datatype it should have the same handles/methods and they should work the same.  But the inner workings of HOW they do this can be different.