# Is the letter delta generally only used to express change in variable or quantity?

I was speaking with a friend of mine earlier and he said "Oh look, delta, the sign of uncertainty" (he doesn't study physics often so had only seen in in Heisenberg's Uncertainty Principle equations). I said "Actually, delta is used to express change in variable or quantity, and is only used for uncertainty in Heisenberg's Uncertainty Principle and other such equations".

When I said "other such equations" I meant the ones that are identical to the most famous Uncertainty Principle equation, which just a change in variable, for example: you can't know both the direction of a rotating object and how fast it's spinning, as well as there being uncertainty in the relationship between the number of photons in a light beam and the phase wave associated with the beam (both are expressed with a similar equation to Heisenberg's Uncertainty Principle and use the delta symbol to represent uncertainty.

Was I wrong? Are there other reasonably well-known equations that use the delta symbol to express a variable that isn't change in variable/quantity or uncertainty?

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Of course it is all convention, but I'd say $\Delta$ is more often an operator than a variable. It has the 'change of' meaning you comment, but is also used for the Laplacian.

The lowercase version, $\delta$, is more often used as a variable, most notably in all those $\delta-\epsilon$ definitions and theorems of calculus. It also has universal meaning as Dirac's delta function or its discrete sister, Kronecker's delta. And in variational calculus it is used to represent the variation of a functional.

And there are probably a bunch of other uses I am missing.

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Well, the delta you mentioned is a convention for expressing such a variable that changes in discrete amount.....( if i m not wrong)....

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## protected by Qmechanic♦Mar 1 '13 at 21:03

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