The term "humanoid" (Latin: "of human form") generally refers to bipedal species, usually sentient, with four limbs: two legs and two arms with hands for grasping and tool manipulation. The head usually contains eyes, ears, a mouth, and a brain.
All humanoids are indirectly descended from a single species, sometimes referred to as the Progenitors, who seeded many planets in the Milky Way galaxy with their DNA approximately four billion years ago. (TNG: "The Chase") Further spreading of humanoid species from world to world was carried out by the Preservers, who relocated members of species they believed to be in danger of extinction. (TOS: "The Paradise Syndrome")
The means of the Progenitors' work was a sophisticated retrovirus designed to introduce humanoid traits into the DNA of a planet's existing life forms. The retrovirus' job was to ensure that once a species emerged that could withstand the injection of its traits, it would continually guide the evolution of the species by renewing those traits, countering the natural evolution that might have taken place otherwise. (Star Trek: Sigils and Unions background essay--"Hybrid Theory")
The observed majority of humanoid species are of mammalian-type forms: warm-blooded, often with fur or hair, but not always. Examples of mammalian species include humans, Bajorans, and Elaysians. Reptilians, generally cold-blooded and often with scales, have also been observed, to include Saurians and Xindi-Reptilian. Falling in between these two groups are the therapsid Cardassians, who exhibit traits of both types--hair and scales both, live birth, but a slightly less warm-blooded metabolism. Avian humanoids include the closely-related Aurelians and Skorrs. Insectoid species with bipedal form similar to the other types of humanoids have been observed as well, such as the Xindi-Insectoids.
Due to the Progenitors' genetic manipulation, certain species are able to produce viable hybrid offspring with each other, though usually with medical assistance.
Humanoids are usually known by other terms in the languages of non-human species. When translated into Federation Standard, for instance, the Cardassians refer to these species as "cardasdanoids." (Star Trek: Sigils and Unions)
List of humanoid species
To view a list of humanoid species, click here.