Was pi invented or discovered?

[Aractus]

"Red" is not an intrinsic property of light, it's merely a way for humans to describe how our eyes perceive certain wavelengths.

Actually, for us it is. The light is processed by our retinas for all intents and purposes as if it was actually made up of RED, GREEN and BLUE. Of course there are still no primary colours, and you're right that colour is just a wavelength, however that is in fact how we see the world, and why colour-blindness typically affects one of the colour processors in our brains.

"Circle" is the name of a defined surface area, which exists in the physical world. Circles have always been there, but the name for them is a human invention.

Incorrect. Circle is the name of a mathematical object. It is relatively easy to produce and generate in mathematics, but doesn't actually exist in the real world due to real-world limitations.

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What is the ratio of circumference/diameter for an imperfect circle called?
3.14159 ?

Lion, try to keep up. "Imperfect circles" are "regular polygons" which have a high number of sides - eg 9,000 sides.

Imperfect circles actually DO exist in real life and they can be assigned an approximate C/D ratio. (Cant they?)

They have an exact C/D ratio - Pi is the "approximate" since it's infinitely imprecise. You calculate it using triangulation, in other words the 9,000 sided regular polygon can be broken down into 9,000 Isosceles Triangles identical to each other. You only need to know the length of the shorter side (let's say it's 1/9,000th of the total "circumference" of your "imperfect circle"). All you need is the angle. The interior angle (ie at the centre of the polygon where the two long sides of the isosceles triangle meet) is equal to 180/9000. The other two angles are (180 - (180/9000))/2. Exactly 89.99 degrees (not a bad number considering I chose 9,000 at random).

The formula you need is: K = 0.5 x c2 x sin(89.99)2/sin(0.02). c = 1/9,000th of your circumference.

Say for example you have a circle, circumference 1500mm. But it's actually a 9,000 sided regular polygon. What's the area?

K = 9000 x 0.5 x (1500/9000)2 x sin(89.99)2/sin(0.02).

Rather complicated. You get the same results using Pi much easier, just knowing the circumference (1500mm):

Area = Pi x ((1500/Pi)/2)2

I hope I didn't make any errors above...