A transcendental
number is a number that cannot be expressed as a
root of a
polynomial equation with
rational coefficients. (Oppose
algebraic).
Some notable transcendental numbers:
Interestingly (to me, anyhow), although e
Pi is known to be transcendental, it is not known whether or not e
e, Pi
e, or Pi
Pi are transcendental.
Reference: Eric Weisstein's World of Mathematics
http://mathworld.wolfram.com/TranscendentalNumber.html