BUILT-IN MATHEMATICA SYMBOL

With

With[{x=x₀, y=y₀, ...}, expr]
specifies that in expr occurrences of the symbols x, y, ... should be replaced by , , ....

Details Details

With allows you to define local constants.
With replaces symbols in expr only when they do not occur as local variables inside scoping constructs.
You can use With[{vars}, body/;cond] as the right-hand side of a transformation rule with a condition attached.
With has attribute HoldAll.
With constructs can be nested in any way, with inner variables being renamed if necessary.
With is a scoping construct that implements read-only lexical variables.