Twilight Sparkle

Salutations, Princess of Equestria, and welcome to our fair forums, created in your honour! If you deign to accept my petition, I, Duality of the Elemental Plane, would ask a humble boon of your singularly majestic personage - specifically, a question pertaining to your field of greatest knowledge.

What is the simplest and most elegant definition of friendship you have ever discovered?