The Collaborative International Dictionary of English v.0.48:

Taproot \Tap"root`\, n. (Bot.)
   The root of a plant which penetrates the earth directly
   downward to a considerable depth without dividing.
   [1913 Webster]

WordNet (r) 3.0 (2006):

taproot
    n 1: (botany) main root of a plant growing straight downward
         from the stem
    2: something that provides an important central source for
       growth or development; "the taproot of his resentment";
       "genius and insanity spring from the same taproot"

Moby Thesaurus II by Grady Ward, 1.0:

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