The Collaborative International Dictionary of English v.0.48:

Atomic \A*tom"ic\, Atomical \A*tom"ic*al\, a. [Cf. F. atomique.]
   1. Of or pertaining to atoms.
      [1913 Webster]

   2. Extremely minute; tiny.
      [1913 Webster]

   Atomic bomb, see atom bomb in the vocabulary.

   Atomic philosophy, or Doctrine of atoms, a system which,
      assuming that atoms are endued with gravity and motion,
      accounted thus for the origin and formation of all things.
      This philosophy was first broached by Leucippus, was
      developed by Democritus, and afterward improved by
      Epicurus, and hence is sometimes denominated the Epicurean
      philosophy.

   Atomic theory, or the Doctrine of definite proportions
      (Chem.), teaches that chemical combinations take place
      between the supposed ultimate particles or atoms of
      bodies, in some simple ratio, as of one to one, two to
      three, or some other, always expressible in whole numbers.


   Atomic weight (Chem.), the weight of the atom of an element
      as compared with the weight of the atom of hydrogen, taken
      as a standard.
      [1913 Webster]

WordNet (r) 3.0 (2006):

atomic
    adj 1: of or relating to or comprising atoms; "atomic
           structure"; "atomic hydrogen"
    2: (weapons) deriving destructive energy from the release of
       atomic energy; "nuclear war"; "nuclear weapons"; "atomic
       bombs" [syn: nuclear, atomic] [ant: conventional]
    3: immeasurably small

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "atomic":
   a certain, an, any, any one, atomatic, atomiferous, atomistic,
   corpuscular, cyclic, diatomic, dibasic, either, embryonic,
   evanescent, exclusive, germinal, granular, heteroatomic,
   heterocyclic, hexatomic, homocyclic, impalpable, imperceptible,
   imponderable, inappreciable, indiscernible, individual,
   indivisible, infinitesimal, intangible, integral, invisible,
   irreducible, isobaric, isocyclic, isoteric, isotopic, lone,
   microcosmic, microscopic, molecular, monadic, monatomic, monistic,
   one, pentatomic, simple, single, singular, sole, solid, solitary,
   subatomic, tenuous, tetratomic, thin, triatomic, tribasic,
   ultramicroscopic, unanalyzable, undivided, uniform, unique,
   unitary, unseeable, whole



The Jargon File (version 4.4.7, 29 Dec 2003):

atomic
 adj.

    [from Gk. atomos, indivisible]

    1. Indivisible; cannot be split up. For example, an instruction may be said
    to do several things ?atomically?, i.e., all the things are done
    immediately, and there is no chance of the instruction being half-completed
    or of another being interspersed. Used esp. to convey that an operation
    cannot be screwed up by interrupts. ?This routine locks the file and
    increments the file's semaphore atomically.?

    2. [primarily techspeak] Guaranteed to complete successfully or not at all,
    usu. refers to database transactions. If an error prevents a
    partially-performed transaction from proceeding to completion, it must be ?
    backed out?, as the database must not be left in an inconsistent state.

    Computer usage, in either of the above senses, has none of the connotations
    that ?atomic? has in mainstream English (i.e. of particles of matter,
    nuclear explosions etc.).


The Free On-line Dictionary of Computing (30 December 2018):

atomic

    (From Greek "atomos", indivisible) Indivisible;
   cannot be split up.

   For example, an instruction may be said to do several things
   "atomically", i.e. all the things are done immediately, and
   there is no chance of the instruction being half-completed or
   of another being interspersed.  Used especially to convey that
   an operation cannot be interrupted.

   An atomic data type has no internal structure visible to the
   program.  It can be represented by a flat domain (all
   elements are equally defined).  Machine integers and
   Booleans are two examples.

   An atomic database transaction is one which is guaranteed to
   complete successfully or not at all.  If an error prevents a
   partially-performed transaction from proceeding to completion,
   it must be "backed out" to prevent the database being left in
   an inconsistent state.

   [Jargon File]

   (2000-04-03)