Twin refers to a sentient avatar, existing solely within the Virtual World. Prior to her death, Zoe Graystone developed software that created her twin avatar, using a technique of datamining. In addition, she programmed the software to allow the avatar to feel similar sensations as the person from which the avatar was modelled.
Zoe's twin avatar was the first known twin, and prior to perfection, the twin de-rezzed, perhaps frequently. Daniel Graystone, with assistance from Zoe's twin, created another twin avatar, modelled after Tamara Adama. Later, Daniel captured Zoe's twin, removing it from the V-World, and accidentally destroying the programming that created it. Currently, Tamara's twin is the only known twin that exists.
Twin avatars appear to be sleepers to other V-World participants, except that the updated programming doesn't cause the avatar to de-rez if the avatar is "killed" or "injured."
The term "twin" is not used on Caprica to refer to this entity.