The term humanoid refers to any lifeform whose body structure resembles that of a Human. In this sense, the term describes sentient bipedal lifeforms in possession of two arms, two legs, hands, feet, one thorax, a neck, and a head. They are generally mammalian but the term is not exclusive to that biological classification.
The majority of the galaxy's intelligent species are humanoid which is no coincidence, as all originate from an earlier species which seeded many worlds with DNA codes which would lead to the evolution of lifeforms similar to themselves. These ancient progenitors, whom it is thought might be precursors of the Preservers, left evidence of their existence in the form of DNA sequences on various planets which when assembled formed a computer program to communicate with their descendants. The first known playing of this message was in 2369 when members of several humanoid species worked together to complete it. (TNG episode: "The Chase"; Last Unicorn RPG module: All Our Yesterdays: The Time Travel Sourcebook)