The Collaborative International Dictionary of English v.0.48:

Fundamental \Fun"da*men`tal\, n.
   A leading or primary principle, rule, law, or article, which
   serves as the groundwork of a system; essential part, as, the
   fundamentals of the Christian faith.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fundamental \Fun`da*men"tal\, a. [Cf. F. fondamental.]
   Pertaining to the foundation or basis; serving for the
   foundation. Hence: Essential, as an element, principle, or
   law; important; original; elementary; as, a fundamental
   truth; a fundamental axiom.
   [1913 Webster]

         The fundamental reasons of this war.     --Shak.
   [1913 Webster]

         Some fundamental antithesis in nature.   --Whewell.
   [1913 Webster]

   Fundamental bass (Mus.), the root note of a chord; a bass
      formed of the roots or fundamental tones of the chords.

   Fundamental chord (Mus.), a chord, the lowest tone of which
      is its root.

   Fundamental colors, red, green, and violet-blue. See
      Primary colors, under Color.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fundamental
    adj 1: serving as an essential component; "a cardinal rule";
           "the central cause of the problem"; "an example that was
           fundamental to the argument"; "computers are fundamental
           to modern industrial structure" [syn: cardinal,
           central, fundamental, key, primal]
    2: being or involving basic facts or principles; "the
       fundamental laws of the universe"; "a fundamental
       incomatibility between them"; "these rudimentary truths";
       "underlying principles" [syn: fundamental, rudimentary,
       underlying]
    3: far-reaching and thoroughgoing in effect especially on the
       nature of something; "the fundamental revolution in human
       values that has occurred"; "the book underwent fundamental
       changes"; "committed the fundamental error of confusing
       spending with extravagance"; "profound social changes" [syn:
       fundamental, profound]
    n 1: any factor that could be considered important to the
         understanding of a particular business; "fundamentals
         include a company's growth, revenues, earnings, management,
         and capital structure"
    2: the lowest tone of a harmonic series [syn: fundamental,
       fundamental frequency, first harmonic]

Moby Thesaurus II by Grady Ward, 1.0:

239 Moby Thesaurus words for "fundamental":
   AF, ab ovo, abecedarian, aboriginal, antenatal, audio frequency,
   austere, autochthonous, axiom, bare, basal, base, basement, basic,
   basilar, basis, bearing wall, bed, bedding, bedrock, beginning,
   bench mark, bottom, budding, cardinal, cardinal point, center,
   central, chaste, chief thing, climax, component, constituent,
   constitutional, constitutive, core, cornerstone, creative, crisis,
   critical, critical point, crucial, crux, dominant, element,
   elemental, elementary, elements, elixir, embryonic, essence,
   essential, essential matter, factor, fetal, first, floor, flooring,
   flower, focal, focus, fond, footing, formative, foundation,
   foundational, frequency, fundament, fundamental tone, generative,
   genetic, germinal, gestatory, gist, grammar, gravamen, great point,
   ground, grounds, groundwork, gut, hardpan, harmonic, heart,
   high point, homely, homespun, homogeneous, hypostasis, important,
   important thing, in embryo, in its infancy, in ovo, in the bud,
   inaugural, inceptive, inchoate, inchoative, incipient, incunabular,
   indispensable, indivisible, infant, infantile, inherent, initial,
   initiative, initiatory, inner essence, intonation, intrinsic,
   introductory, inventive, irreducible, issue, kernel, keystone,
   landmark, law, life-and-death, life-or-death, main, main point,
   main thing, marrow, material, material point, meat, mere,
   milestone, monolithic, monotone, monotony, nascent, natal,
   necessary, needful, nub, nucleus, nuts and bolts, of a piece,
   of the essence, of vital importance, organic, original, overtone,
   paramount, part and parcel, partial, partial tone, parturient,
   pavement, pitch, pith, pivot, plain, postnatal, postulate,
   pregnant, prenatal, primal, primary, prime, primeval, primitive,
   primogenial, primordial, principal, principium, principle,
   principles, pristine, procreative, protogenic, pure,
   pure and simple, quid, quiddity, quintessence, quintessential,
   radical, real issue, requisite, riprap, rock bottom, rudiment,
   rudimental, rudimentary, rudiments, rule, salient point, sap, seat,
   seminal, severe, sill, simon-pure, simple, sine qua non, single,
   solid ground, solid rock, soul, spare, spirit, stark, stereobate,
   stuff, stylobate, substance, substantial, substantive,
   substantive point, substratum, substruction, substructure,
   terra firma, the bottom line, the nitty-gritty, the point, theorem,
   tone, tonelessness, turning point, unadorned, uncluttered,
   underbuilding, undercarriage, undergirding, underlying,
   underpinning, understruction, understructure, undifferenced,
   undifferentiated, uniform, ur, vital