A cogenitor
A cogenitor is one of the three genders of the Vissian species and is required for a male and female Vissian to successfully reproduce. The cogenitor does not pass on genetic material to the offspring they help create; Dr. Phlox suggested that they may supply an enzyme during the sex act which facilitates conception.
Cogenitors make up only three percent of the Vissian population and their services for reproduction are consequently in high demand. However, in contrast to their rarity, Cogenitors are not assigned a high position in Vissian society. Until at least the mid-22nd century they were relegated to absocial status far inferior to that of the male and female genders. Cogenitors were not allowed to learn to read or write, were often dressed in the shabbiest of clothing, and had no input as to what couples they would help procreate or when. They were not even given a name. (ENT: "Cogenitor")