From Memory Alpha, the free Star Trek reference.
Data crystals are small, sophisticated information storage devices able to store a large amount of information. They can be connected to
dataports. Prior to
Arissa's undercover assignment to the
Orion Syndicate, which required her memories to be altered, a copy of her memories were stored inside a data crystal so that her identity could be restored after the assignment was complete. (
DS9: "
A Simple Investigation")
