World Seed

The «World Seed» (世界の種子), is an item that holds a program package called «The Seed» (ザ・シード). The Seed is a free preset series of program packages that can be used for creating Virtual Reality worlds as well as control the input and output of the five senses of the players, allowing players to FullDive into the created world. The Seed was created by Kayaba Akihiko and replaced usual proprietary game systems like the Cardinal System that Kayaba had developed while working for Argus which was required to run VR games, but had a terrifyingly huge license fee.

Background
It was given to Kirito by a computerized memory of Kayaba Akihiko, who wanted The Seed to sprout and grow, after Kirito had saved Asuna from the clutches of Sugou. Although Kayaba allows Kirito to choose what to do with it, after confirming that The Seed is safe to use, Kirito 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 VR worlds.

System
«The Seed» includes the basic programs needed to create a VR world, such as the audio and video system, as well as a downsized version of the Cardinal System, which was used in Sword Art Online to help in the development and to maintain control 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 that 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 allow 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 or her data to another game and keep their stats instead of starting from scratch, but all the items and money from the previous game will be lost, unless they are given to another player before the transfer for safekeeping. The whole interconnected network is known as The Seed Nexus.

Trivia

 * Although Underworld was created using the public version of The Seed which does not include Sword Skills, they were somehow implemented in the system.