Ultranauts were a faction of transhumans who served as workers and rulers aboard lighthuggers.
The majority of Ultras opted for extensive and obvious mechanical modifications, replacing their original limbs and organs. But while this was their most obvious and apparently widespread trend, not all Ultras utilised such modifications. Ilia Volyova had no obvious modifications and was still considered -- by herself and others -- an Ultra. Some Ultras, conversely, modified themselves in less obvious ways, looking fully human yet crawling with internal machinery.
Ultras tended to have lived long lives, partly due to long periods of cryo-preservation or "reefersleep" during interstellar transit, partly due to the time-dilating effects of near-lightspeed travel, and partly due to their willingness to replace failing organs and limbs with mechanical or bio-engineered alternatives.
A quirk of Ultra society was that for each session in reefersleep, they grew and maintain a dreadlock as a badge of their status, although they were also known to use these status symbols as stakes in gambling games; once again, however, there were exceptions like Ilia Volyova, who -- although an Ultra -- did not have dreadlocks. Aesthetics did appear to be of greater concern to most Ultras than more pragmatic concerns such as functionality and reliability. Some of the more extensively modified Ultras had, apparently intentionally, turned themselves into living sculptures.
Unlike other factions, such as the Conjoiners and Demarchists, there was no unifying political structure or philosophical school of thought behind Ultra society. Although extremely isolated from the rest of humanity during their long voyages, they did not appear to form particularly close associations even within crews; for the crews of the Nostalgia for Infinity, the Orvieto, and the Gnostic Ascension, power struggles and mistrust were a normal state of affairs. However, crews such as those were not exactly the norm; Inigo Standish, the shipmaster of the Petronel, held that Ultras like those were a minority.
Ultranauts in the Revelation Space series
Major and named Ultranaut characters appearing in the series, sorted by their home vessel.
- Nostalgia for Infinity
RS Glossary entry
Ultranauts: The permanent starfaring crew of lighthuggers.
- description from the official RS glossary 
- Revelation Space universe - RS Glossary - U, AlastairReynolds.com, the official website of Alastair Reynolds
- Lighthugger - The most advanced human starships in the Revelation Space series, usually owned and operated (but not developed and constructed by) Ultranauts.