Androgyny

Androgyny is the absence of gender, or the possession of qualities of both genders.

In May 2151, the crew of discovered that the Axanar were a long-lived androgynous species. 

The Alpha Quadrant species J'naii were androgynous. 

Morn was referred to as androgynous by Odo, who while making an announcement to passers-by at Quark's of Deep Space 9 says, "Ladies and gentlemen (notices Morn walking by) ...and all androgynous species...". 

Fantome's species appeared to be androgynous and they lacked any form of known gender identifying features.