An atom is the following:

• Generally, an atom is the smallest irreducible constituent of a chemical system.
• In Prolog, an atom is a specific value (as opposed to an unbound variable, which represents all possible values).
• The Japanese name, Atom is the name of the popular animated character known in English as Astroboy.
• In web syndication, Atom is a standard XML format.
• An atom is a non-zero member of a Boolean algebra that is not a join of any other elements.
• Atom: Journey Across the Subatomic Cosmos is a book by Isaac Asimov.
• The Acorn Atom was an early eighties home computer.
• An atom or atomic value is an element of a programming language that is considered to have no parts -- a single unit. Most atoms are single characters, such as + or 3; but some atoms are built of several characters, such as MediaWiki's subst or pascal's begin and end.