The Collaborative International Dictionary of English v.0.48:

Basis \Ba"sis\ (b[=a]"s[i^]s), n.; pl. Bases (b[=a]"s[=e]z).
   [L. basis, Gr. ba`sis. See Base, n.]
   1. The foundation of anything; that on which a thing rests.
      --Dryden.
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   2. The pedestal of a column, pillar, or statue. [Obs.]
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            If no basis bear my rising name.      --Pope.
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   3. The groundwork; the first or fundamental principle; that
      which supports.
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            The basis of public credit is good faith. --A.
                                                  Hamilton.
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   4. The principal component part of a thing.
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WordNet (r) 3.0 (2006):

basis
    n 1: a relation that provides the foundation for something;
         "they were on a friendly footing"; "he worked on an interim
         basis" [syn: footing, basis, ground]
    2: the fundamental assumptions from which something is begun or
       developed or calculated or explained; "the whole argument
       rested on a basis of conjecture" [syn: basis, base,
       foundation, fundament, groundwork, cornerstone]
    3: the most important or necessary part of something; "the basis
       of this drink is orange juice" [syn: basis, base]