Virtualization

Virtualization is the process of an entity being transferred onto Lyoko.

The process was first chronologically used on humans with Waldo Schaeffer and his daughter, Aelita, for them to escape to Lyoko from the men in black. It may have been invented by Waldo.

The process is usually run by Jeremie, Aelita, or by the Self Virtualization Program.

Description

The only possible way (shown in the series) to do this is to use the scanners. The process is not fully detailed but it appears to be broken into stages.

The first stage is avatar recognition. Jeremie selects an avatar and the computer realizes the stored genetic structure of the Lyoko Warrior.
The second stage is scanning. The person's molecular structure is identified and stored if the person is being virtualized for the first time; if the Supercomputer already recorded their virtualization beforehand it will simply be recognized.
The third stage is the final virtualization; this consists of the character's "digitization" (conversion from physical matter into digital matter) and finally, their appearance on Lyoko.

It has been revealed in Code Lyoko Evolution that people may only be virtualized within 12-hour intervals of their previous virtualization. This serves as a retcon because, in the first four seasons, it didn't seem to take that long in order to be sent back to Lyoko. For example, in the episode Common Interest, Odd was virtualized with Ulrich to do a sweep of the Ice Sector to locate an activated tower. Jeremie brought both of them back to pick up Aelita who was recovering in the hospital. Odd was then virtualized again with Aelita and Yumi not much later in the same day to help Aelita deactivate the tower.

Another example of this would be in Dog Day Afternoon, where Odd was virtualized with Aelita to the Desert Sector to cover her while she enters the tower that was controlling the William Clone to install new security features in it. After Jeremie brought them both back to Earth, not much later in that same exact day, Odd was re-virtulalized to the Mountain Sector so Jeremie could try and undo Odd's fusion with Kiwi (who was in the scanner with Odd when he went to the Desert Sector with Aelita, unbeknownst to both her and Jeremie). Afterwards, when the Superscan detected an activated tower in the Ice Sector, Aelita was sent back to Lyoko with Ulrich to deactivate it.

It hasn't been made clear why this 12-hour rule is apparent in Evolution, but this major retcon has served as a problem for the Lyoko Warriors in fighting X.A.N.A.. It's possible that being materialized while retaining life points allows one to be virtualized again.

This process can have mistakes, both due to operator error and machine error. Instances of this include:

In Frontier, Yumi virtualized Jeremie incorrectly, accidentally dumping him into the Virtual Limbo.
In Dog Day Afternoon, Odd brought Kiwi in the scanner with him, causing their DNA to merge.
Due to outside interference in the Virtualization Enhancement Program, in Kadic Bombshell, people were virtualized into the wrong sectors.