Dax (commonly known as The Old Man) was a Trill, a continuation of the line of hosts of the Dax symbiont. By the 25th century, the Dax symbiont lived in a male host, and was considered a valuable source of information regarding the days of the 24th century Dominion War, due to the key role Jadzia Dax and Ezri Dax played in the conflict. (STO website: Season 5 News)