Definition:Definitional Abbreviation

Definition
When discussing a formal language, some particular WFFs may occur very often.

If such WFFs are very unwieldy to write and obscure what the author tries to express, it is convenient to introduce a shorthand for them.

Such a shorthand is called a definitional abbreviation.

It does not in any way alter the meaning or formal structure of a sentence, but is purely a method to keep expressions readable to human eyes.