The Collaborative International Dictionary of English v.0.48:

Radix \Ra"dix\ (r[=a]"d[i^]ks), n.; pl. L. Radices
   (r[a^]d"[i^]*s[=e]z), E. Radixes (r[=a]"d[i^]ks*[e^]z). [L.
   radix, -icis, root. See Radish.]
   1. (Philol.) A primitive word, from which spring other words;
      a radical; a root; an etymon.
      [1913 Webster]

   2. (Math.)
      (a) A number or quantity which is arbitrarily made the
          fundamental number of any system; a base. Thus, 10 is
          the radix, or base, of the common system of
          logarithms, and also of the decimal system of
          numeration.
      (b) (Alg.) A finite expression, from which a series is
          derived. [R.] --Hutton.
          [1913 Webster]

   3. (Bot.) The root of a plant.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "radix":
   beginning, bulb, bulbil, commencement, conception, corm,
   derivation, genesis, grass roots, head, inception, origin,
   original, origination, provenience, radical, radicle, rhizome,
   rise, root, rootstock, source, stem, stock, tap, taproot, tuber,
   tubercle