Androgyny, as well as neuter, were terms used to indicate the absence of gender. Androgynous species had uniform and identical sexual characteristics and functions for the creation of offspring. Although simpler lifeforms, like amoebae, simply divided their genetic material to procreate, more advanced lifeforms like Xindi-Insectoids were genderless and reproduced asexually. (ENT: "Hatchery")
The Bynars were also a species that did not follow the traditional two-gender norm. When Commander William T. Riker first encountered the species on Starbase 74, he inquired to their handler, Commander Quinteros, "are these gentlemen the Bynars?" Quinteros explained that "they're not gentlemen, or ladies, commander. They are a unified pair. They're always together." Riker, nevertheless, continued to refer to them as "gentlemen", after being impressed with their creation of Minuet. (TNG: "11001001")
During Q's introduction of the Borg to the Federation, he pointed out, "Interesting, isn't it? Not a he, not a she. Not like anything you've ever seen. An enhanced humanoid." However, after Captain Jean-Luc Picard attempted to communicate with the Borg drone that had invaded the USS Enterprise-D's main engineering, Q changed his tune, adding, "You're nothing to him. He's not interested in your lifeform. He's just a scout, the first of many. He's here to analyze your technology. He may attempt to gain control of the ship. I wouldn't let him." (TNG: "Q Who")
The J'naii reproduced by recombining the genetic material of two individuals, but without assigning either of them a gender role in the sexual process. They had evolved to become this way from a two-sex state. A few like Soren still identified as either male or female, though they were viewed as "deviants" and then subjected to psychotectic treatment if discovered. (TNG: "The Outcast")
Morn was once 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")
In the Star Trek: Enterprise episode "The Crossing", when Malcolm Reed was taken over by non-corporeal "wisps," he reminded a crewman that she was a female, and later told T'Pol that she was female, and that he wished to mate with her, suggesting that the concept of gender was unfamiliar to them. A similar lack of understanding genders was also indicated by Nagilum in the Star Trek: The Next Generation episode "Where Silence Has Lease", suggesting their lack of gender.
Several other species to appear in Star Trek had appearances that suggested their being androgynous (mostly in cases where female actresses portrayed male-type characters), but were otherwise indicated to be male, such as the Talosians and the Metrons.
Fantome's species also appeared to be androgynous and they lacked any form of known gender-identifying features.