YAGNI on Wikipedia
This is an acronym for You Ain't Gonna Need It.
Always implement things when you actually need them, never when you just foresee that you need them.
(Ron Jeffries) (XP co-founder and author of the book "Extreme Programming Installed")
This Extreme Programming (XP) principle suggests developers should only implement functionality that is needed for the immediate requirements, and avoid attempts to predict the future by implementing functionality that might be needed later.
Adhering to this principle should reduce the amount of unused code in the codebase, and avoid time and effort being wasted on functionality that brings no value.