A taproot is an enlarged somewhat straight to tapering plant root that grows vertically downward. It forms a center from which other roots sprout laterally. Plants with taproots are difficult to transplant. The presence of a taproot is why dandelions are hard to uproot — the top is pulled, but the long taproot stays in the ground and re-sprouts. The taproot system contrasts to the fibrous root system with many branched roots.
Definition adapted from URL: http://en.wikipedia.org/wiki/Taproot