The DRY Principle

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

DRY is an acronym for Don't Repeat Yourself. This principle aims to help developers reducing the repetition of code and keep the information in a single place and was cited in 1999 by Andrew Hunt and Dave Thomas in the book The Pragmatic Programmer

The opposite of DRY would be WET (Write Everything Twice or We Enjoy Typing).

In practice, if you have the same piece of information in two (or more) different places, you can use DRY to merge them into a single one and reuse it wherever you want/need.