Lisp atom
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In the original LISP, the originator of the Lisp programming language family, there were two fundamental data types, atoms and lists. A list was a finite ordered sequence of elements, where each element is in itself either an atom or a list, and an atom was a number or a symbol. A symbol was essentially a unique named item, written as an alphanumeric string in source code, and used either as a variable name or as a data item in symbolic processing. For example, the list (FOO (BAR 1) 2) contains three elements: the symbol FOO, the list (BAR 1), and the number 2.
The essential difference between atoms and lists was that atoms were immutable and unique. Two atoms that appeared in different places in source code but were written in the exact same way represented the same object, whereas each list was a separate object that could be altered independently of other lists and could be distinguished from other lists by comparison operators.
As more data types were introduced in later Lisp dialects, and programming styles evolved, the concept of an atom lost importance. Many dialects still retained the predicate atom for legacy compatibility, defining it as true for anything that is not a cons cell (ie. a list or a partial list).