The Golden Mean (aka the
Golden Section) is an
irrational number like Pi, or e (not the
drug you dance-music crazed goose! e, the base of all natural
logarithms).
It is given the greek symbol Phi (ANSI character 237 - looks like a circle with a slash through it). Its value is sqrt(1.25) + 0.5, or approximately 1.61803398875
If you take a square, draw a line from the middle of one of its sides to an opposite corner, and rotate that line so it is in line with the bisected side, and use that to make a rectangle, the ratio of the long side to the short side is the Golden Mean.
The classical Greeks said that objects fashioned using this proportion were more pleasing to the eye. This is probably because Phi occurs in nature - in seashell spirals, packed objects, etc. Indeed, the Greeks used the Golden Section a lot in their architecture.
Among other interesting features of the Golden Section is that its reciprocal is 0.61803398875, or Phi minus one. This value is given the symbol phi (lower case "p").