Androgyny is the absence of gender, or the possession of qualities of both genders.
In May 2151, the crew of Enterprise (NX-01) discovered that the Axanar were a long-lived androgynous species. (ENT: "Fight or Flight")
The Alpha Quadrant species J'naii were androgynous. (TNG: "The Outcast")
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...". (DS9: "If Wishes Were Horses")
Even though Morn and members of his species have been referred to as males and females in subsequent episodes, it is possible the androgyny in his race is that of gender identity. It is also possible Odo was making a joke at Morn's expense, though given Odo's apparent lack of a sense of humor, he might have been serious.
Fantome's species appeared to be androgynous and they lacked any form of known gender identifying features. (VOY: "The Void")