An Etymological Dictionary of Astronomy and Astrophysics

1. Math.: A quantity that, when multiplied by itself a certain number of times, produces a given quantity. For example, since 3 × 3 × 3 × 3 = 81, 3 is a fourth root of 81.
   

2. Math.: A solution to an equation f(x) = 0. We say that x₀ is a root or zero of a → polynomial if f(x₀) = 0. For example, the roots of the equation x² - 9 = 0 are +3 and -3.
   

3. In → graph theory, a special → vertex that turns a → tree into a → rooted tree or a graph into a → rooted graph.

Etymology (EN): From M.E., from O.E. rot, from O.N. rot “root;” cf. O.H.G. wurz “plant, herb;” Ger. Wurz; cognate with L. radix, radius “staff;” Gk. rhiza “root;” Albanian rrânzë “root;” PIE base *u(e)rad- “twig, root.”

Etymology (PE): Rišé “root” (dialectal Tabari rexa; Kurd. regez, riše), from Mid.Pers. rêšak “root,” maybe ultimately related to PIE *u(e)rad-, as above, although the Skt. offshoot is absent.