Twin refers to a sentient avatar which exists 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 a biofeedback function into the software to allow the avatar to feel similar sensations as the person from whom the avatar was modelled. Zoe's twin avatar is the first known twin, and prior to perfection, the twin de-rezzed, perhaps frequently. ("Pilot")
Daniel Graystone first meets Zoe's twin in the V-Club several weeks after his daughter's death. He considers her to be an avatar, a copy of Zoe and nothing more. She tells him she does not feel like a copy. She says she is like Zoe's twin sister, like they are echoes of one another.
Later, with the unwitting assistance of Lacy Rand, Daniel captures Zoe's twin, removing it from the V-World. He then uses the Zoe Avatar program to create another twin avatar, modelled after Tamara Adama.
When Daniel uploads Zoe's avatar program into the U-87 robot via the MCP, the transfer briefly works, then fails. Daniel frantically searches the V-World black box, where he kept her twin avatar, only to find that the program is gone - accidentally destroyed. ("Pilot")
Twin avatars appear to be sleepers to other V-World participants, except that the updated programming does not cause the avatar to de-rez if the avatar is "killed" or "injured." ("There Is Another Sky")
Notes
Although the term "twin" is coined from the Zoe Avatar's description of herself, this term is not used on Caprica to refer to this entity.