Reefersleep is a state of cryo-preservation assisted hibernation employed by Ultranauts and others intending to travel through space for extended periods of time.
RS Glossary entry
Reefersleep: The cryogenic freezing technology adopted by Ultras.
- description from the official RS glossary 
- Revelation Space universe - RS Glossary - R, AlastairReynolds.com, the official website of Alastair Reynolds