Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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"