The Collaborative International Dictionary of English v.0.48:

Elementary \El`e*men"ta*ry\, a. [L. elementarius: cf. F.
   ['e]l['e]mentaire.]
   1. Having only one principle or constituent part; consisting
      of a single element; simple; uncompounded; as, an
      elementary substance.
      [1913 Webster]

   2. Pertaining to, or treating of, the elements, rudiments, or
      first principles of anything; initial; rudimental;
      introductory; as, an elementary treatise.
      [1913 Webster]

   3. Pertaining to one of the four elements, air, water, earth,
      fire. "Some luminous and fiery impressions in the
      elementary region." --J. Spencer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

elementary
    adj 1: easy and not involved or complicated; "an elementary
           problem in statistics"; "elementary, my dear Watson"; "a
           simple game"; "found an uncomplicated solution to the
           problem" [syn: elementary, simple, uncomplicated,
           unproblematic]
    2: of or pertaining to or characteristic of elementary school or
       elementary education; "the elementary grades"; "elementary
       teachers"
    3: of or being the essential or basic part; "an elementary need
       for love and nurturing" [syn: elementary, elemental,
       primary]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "elementary":
   ab ovo, abecedarian, aboriginal, acid, alkali, antenatal, austere,
   autochthonous, bare, basal, basic, basilar, beginning, biochemical,
   budding, central, chaste, chemical, chemicobiological,
   chemicoengineering, chemicomineralogical, chemicophysical,
   chemurgic, clear, component, constituent, constitutive,
   copolymeric, copolymerous, creative, crucial, dimeric, dimerous,
   easy, electrochemical, elemental, embryonic, essential, fetal,
   formative, foundational, fundamental, generative, genetic,
   germinal, gestatory, gut, heteromerous, homely, homespun,
   homogeneous, in embryo, in its infancy, in ovo, in the bud,
   inaugural, inceptive, inchoate, inchoative, incipient, incunabular,
   indivisible, infant, infantile, initial, initiative, initiatory,
   integral, integrant, introductory, inventive, irreducible,
   isomerous, macrochemical, material, mere, metameric, monolithic,
   monomerous, nascent, natal, nonacid, of a piece, of the essence,
   original, parturient, photochemical, physicochemical,
   phytochemical, plain, polymeric, postnatal, prefatory, pregnant,
   preliminary, prenatal, primal, primary, prime, primeval, primitive,
   primogenial, primordial, pristine, procreative, protogenic, pure,
   pure and simple, radical, radiochemical, rude, rudimental,
   rudimentary, seminal, severe, simon-pure, simple, single, spare,
   stark, straightforward, substantial, substantive, thermochemical,
   unadorned, uncluttered, uncomplicated, underlying, understandable,
   undifferenced, undifferentiated, uniform, unsubtle, ur