The offspring of two animals, plants, or sentient species of different breeds or races.
Not all sentient species can cross-breed to create a hybrid. For instance: Copper-blood (green) species such as Vulcans, Romulans, and Orions would not mix with Cobalt-blood (blue) species like Andorians or Bolians.
Iron-blood (red) species such as Bajoran, Betazoid, Klingon, and Human, would need to have some sort of genetic assistance to mix with other species (remember, Spock’s mother had genetic assistance to conceive him). This makes Human/Vulcan or Human/Romulan mix canon, but only if the parents had genetic assistance. The exception would be Klingon/Romulan, which is a canon hybrid (ask Paramount how that was able to take place).
Some sentient hybrids include:
These hybrids are possible with medical help: