[icode]

What means the "d" in those formulas?

[aqme28]

The differential. It's an operator meaning "infinitely small slice of ..."

Since the Integral symbol represents an infinite sum, Integral( (f(x)*dx) ) is an infinite sum of infinitely small x-wise slices of f(x), thus giving you the area under the curve f(x).

[Natsu]

Here's a good explanation of what it means: http://mathforum.org/library/drmath/view/60949.html

[rmc]

It's a notation for integrals, part of calculus. The "d" doesn't refer to anything http://en.wikipedia.org/wiki/Integral

[est]

delta

[alex1]

Actually, it means differential.

[lclarkmichalek]

While delta is technically wrong, it is correct in that dx represents delta x, where delta x tends to 0.

[manojlds]

Delta x is represented as Δx