A taproot is an anchoring root growing downward in a vertical orientation and they are the main root anchoring system that the feeding roots grow from. A taproot is usually a thick root that tapers gradually. Taproots are the thick primary root that normally grows straight down under the soil.
Merriam-Webster Online Dictionary
taproot (noun)1.
a primary root that grows vertically downward and gives off small lateral roots
2.
the central element or position in a line of growth or development
Primary root, main root, central root, dominant root, principal root