Mega Evolution

Mega Evolution (メガシンカ Mega Evolution) is a form of Pokémon transformation that is unique, differing from the Formes some Pokémon can undertake. Originating in the Kalos region, where Mega Evolution is the most common, it has eventually spread to other regions after it became widely known. Mega Evolutions are identified by having "Mega" in front of their name, and they are believed to be elevated to a power class similar to Legendary Pokémon.

Overview
In order to undergo a Mega Evolution, the Pokémon must make contact with a Mega Stone, and, in addition, possess a strong bond with their trainer. In addition, an object called a Key Stone can be embedded into nearly anything — a ring, glove, necklace — and resonate with the Mega Stone to induce Mega Evolution, though the customization is not necessary. The feelings of the trainer resonate with the Mega and Key Stone, causing the target Pokémon to undergo a change — to Mega Evolve.

Unlike the Formes some Pokémon can use, several of which are merely cosmetic and don't enhance the battle capability of the Pokémon, a Mega Evolution releases all of the Pokémon's latent energy, theorised by many researchers to be the Aura of the Pokémon, and releases it all at once, inducing a form made purely for combat. A Mega-Evolved Pokémon, as such, has enhanced stats, and in some cases, altered-types as well as new abilities either unique to their forms, or abilities they would not normally gain naturally, such as Mega Charizard Y possessing the fabled ability Drought, only seen in the legendary Pokémon Groudon, and very rarely in Ninetales, a Pokémon regarded to have mythical power. Unlike standard evolution, Mega Evolution can be cancelled by the Pokémon themselves after the battle or task for which it was activated. Lucario is believed to be the first Pokémon to Mega Evolve, lending further credence to the belief that Aura is a factor in Mega Evolution.

A Pokémon can have up to two different Mega Forms, and are categorised as X (ズ X) for the evolution which is either more offensive, or focuses on physical skills, and Y (イ Y) for the more defensive or focuses on special skills.

Mega and Key Stones
A Mega Stone (メガストーン Mega Stone) is a unique type of Evolutionary stone. Mega Stones, along with the other various-types of Evolutionary stones, when, the Life Pokémon, spread it's light across the Kalos region, imbuing these stones with it's powerful life energy, transforming them into the various Evolutionary stones known around the world — and most importantly, Mega Stones used to induce Mega Evolution. Mega Stones, together with Key Stones, resonate with each other, and the bond trainers have with their Pokémon, to release the latent energy the Pokémon has an induce a transformation, referred to as Mega Evolution.

Mega Stones are only found in the Kalos Region, due to their unique origins pretaining to Xerneas and it's life-giving light energy. However, Mega Stones are being imported by other regions after their useage outside of Kalos became known from the Kanto Champion Red and his Charizard. Trainers who visit Kalos also tend to bring Mega Stones back with them; ensuring the Stones leave the region and have become known throughout the world. Before the Stones were made known outside Kalos, the Hoenn Champion, Steven Stone, took it upon himself to begin studying them after finding Aggronite on a trip to Kalos.

Trivia

 * Mega Evolution is the first true "transformation" usable in the main Pokémon series.