The Collaborative International Dictionary of English v.0.48:

Rudiment \Ru"di*ment\ (r[udd]"d[i^]*ment), n. [L. rudimentum,
   fr. rudis unwrought, ignorant, rude: cf. F. rudiment. See
   Rude.]
   1. That which is unformed or undeveloped; the principle which
      lies at the bottom of any development; an unfinished
      beginning.
      [1913 Webster]

            but I will bring thee where thou soon shalt quit
            Those rudiments, and see before thine eyes
            The monarchies of the earth.          --Milton.
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            the single leaf is the rudiment of beauty in
            landscape.                            --I. Taylor.
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   2. Hence, an element or first principle of any art or
      science; a beginning of any knowledge; a first step.
      [1913 Webster]

            This boy is forest-born,
            And hath been tutored in the rudiments
            of many desperate studies.            --Shak.
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            There he shall first lay down the rudiments
            Of his great warfare.                 --Milton.
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   3. (Biol.) An imperfect organ or part, or one which is never
      developed.
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The Collaborative International Dictionary of English v.0.48:

Rudiment \Ru"di*ment\, v. t.
   To furnish with first principles or rules; to insrtuct in the
   rudiments. --Gayton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rudiment
    n 1: the elementary stages of any subject (usually plural); "he
         mastered only the rudiments of geometry" [syn: rudiment,
         first rudiment, first principle, alphabet, ABC,
         ABC's, ABCs]
    2: the remains of a body part that was functional at an earlier
       stage of life; "Meckel's diverticulum is the rudiment of the
       embryonic yolk sac"

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "rudiment":
   Anlage, base, basement, basic, basis, bearing wall, bed, bedding,
   bedrock, blastula, bud, egg, element, elements, embryo, fetus,
   floor, flooring, fond, footing, foundation, fundament, fundamental,
   fundamentals, germ, germen, grammar, ground, grounds, groundwork,
   hardpan, larva, loins, nucleus, nymph, ovum, part and parcel,
   pavement, principle, principles, radical, riprap, rock bottom,
   seat, seed, sill, solid ground, solid rock, spermatozoon,
   stereobate, stylobate, substratum, terra firma, underbuilding,
   undercarriage, undergirding, underpinning, understruction,
   understructure, zygote