World Seed

The «World Seed» was given to Kirito by a computerized memory of Kayaba Akihiko after he had saved Asuna from the clutches of Oberon. He wanted Kirito to plant the seed and let it sprout.

Background
«World Seed» is a program that allows the creation of VRMMORPG. It was originally developed by Kayaba Akihiko and was primarily used in the creation of Sword Art Online. It was given to Kirito to decide what to do with it, so he decides to publish it (with Agil's help) on a server so everyone can download it for free. Using the World Seed as a base, cyber technicians can further develop the VR world.

System
The seed includes the base programs need to create a VR world, such as the audio and video system, as well as a "dumbed down" version of the Cardinal program used in SAO to help in the development of the game. Some of the features include automatic quest generation based on myths, legends or stories of the real world. It can create an infinite number of quests, and can even modify entire maps from the game. It is made so the created VR world does not require one large server like the original SAO but can run on multiple smaller servers.

All games using the seed as their game engine allows conversion of one account in a seed-based VR world to another. For example, this ability is used by Kirito when he converts his account from Alfheim Online to Gun Gale Online and back. This allows a player to transfer his data to another game and keep his stats instead of starting from scratch, but all the items and money from the previous game will be lost.