Assigning commonly used formulas a function name

[The Phil]

I know this is possible because I've done it before, but google is not helping me out on this.

There is a way to save commonly used formulas as a command. For example, if I used sum(R6:R8) a lot either as a formula or as a part of a formula, I could save it as the function SR and then use that instead. Can someone remind me how to do this?

Thanks.