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atom

n.
US //ˈætəm// UK //ˈætəm// atom
  1. 1 chemical unit (n.)
    B1 Intermediate Science

    the smallest part of a chemical element that can exist.

    the basic unit of a chemical element, consisting of a nucleus of protons and neutrons typically surrounded by electrons.

    Example

    A water molecule is made of two hydrogen atoms and one oxygen atom.

    Example

    The researchers used a scanning tunneling microscope to manipulate individual atoms on the surface of the silicon crystal.
  2. 2 smallest unit of something (n.)
    C1 Advanced Academic

    the smallest possible piece or unit of an idea or a system.

    the smallest, indivisible constituent part or unit of a larger conceptual or physical system.

    Example

    In this theory, the individual person is the atom of society.

    Example

    Logical atomism suggests that the world consists of ultimate facts that cannot be further decomposed into simpler components.
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  1. 3 a tiny amount (n.)
    C2 Proficiency Literary

    a very small amount of something.

    an extremely small or negligible quantity of a quality or substance.

    Example

    There was not an atom of truth in the rumors spreading through the village.

    Example

    He faced the accusations with dignity, showing not an atom of fear even as the verdict was read aloud.
  2. 4 programming/math (n.)
    C1 Advanced Technical Computing

    a single value or symbol that cannot be broken into a list.

    a primitive value or a scalar that is not a collection; in some languages, a unique integer representing a string.

    Example

    In the Lisp programming language, an atom is a basic element like a number or a symbol.

    Example

    The function is designed to process each atom within the nested list structure recursively until it reaches the base case.
Origin

From Middle English attome, from Middle French athome, from Latin atomus (“smallest particle”), from Ancient Greek ἄτομος (átomos, “indivisible”), from ἀ- (a-, “not”) + τέμνω (témnō, “to cut”, o-grade in τομ-) + -ος (-os). Atoms are so named because they were historically thought up as to be the smallest unit of matter, and thus indivisible. Doublet of atomus.

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