display | more...
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 ePi is known to be transcendental, it is not known whether or not ee, Pie, or PiPi are transcendental.

Reference: Eric Weisstein's World of Mathematics
http://mathworld.wolfram.com/TranscendentalNumber.html