Hybrid

A hybrid either describes:
 * The interspecies reproduction between two or more animals, alien races, or plants; or
 * The intraspecies reproduction between two or more races or ethnic populations within the same species.

As most humanoids share a common genetic ancestor, the s who seeded the Milky Way Galaxy with genetic material, interspecies reproduction can occur between many races. Interspecies reproduction can be difficult between some species as not all species have their s in the same place. 

Starfleet Academy offered an course, and various doctors and scientists continuously researched the topic. 

Hybridization is not without risk. Several anatomical, physiological and psychological abnormalities can occur. Kowalczyk's Syndrome affects the nervous system in hybrids from two or more telepathic species. (Star Trek: Shadowstar Station: "Through the Looking Glass") Vulcan hybrids could suffer from Vulcan Hybrid Societal Maladjustment Syndrome.

Some species are also nearly or totally incompatible. Interspecies reproduction between some races requires genetic engineering to assist the process. Bolians have very different blood chemistry than most other species, and extremely different blood chemistry than Vulcans. Sexual relations between a Bolian and a human would have unfavorable symptoms for the human, including fatigue, nausea and joint inflammation. 

Canon

 * Spock: half-human, half-Vulcan ()
 * Deanna Troi: half-human, half-Betazoid ()
 * Sela: half-human, half-Romulan: ()
 * Tora Ziyal: half-Bajoran, half-Cardassian ()
 * B'Elanna Torres: half-human, half-Klingon ()
 * Naomi Wildman: half-human, half-Ktarian ()

Kal-Dixas Spaceport

 * Jay Allan: half-human, one-quarter Betazoid, one-quarter Vulcan

Star Trek: Black Hawk

 * The Cha'ouw race: Klingon/Romulan hybrids
 * Stormclaw: half-human, one-quarter Klingon, one-quarter Romulan

Star Trek: Hidden Frontier

 * Jenna McFarland: half-human, half-Trill

Star Trek: Marine

 * Brian Kast: half-human, half-Reman

Star Trek: Pendragon

 * Timothy Sinclair: half-human, half-Betazoid
 * Jeffrey, Tyler and Cody Sinclair: three-quarters human, one-quarter Betazoid
 * Varen Sinclair: three-eighths human, one-eighth Betazoid, one-quarter Orion, one-quarter Trill
 * Alareh: half-Orion, half-Trill
 * Jesse Erixon: half-human, half-El Aurian
 * Ciaran Shean: half-Betazoid, half-Rutian
 * Savin th'Vayhr: half-Andorian, half-Vulcan
 * Tr'Aarikah: half-Vulcan, half-Romulan

Star Trek: The Cantabrian Expeditions

 * Scott Fack: M'Tar-altered human
 * Tawana Killan: one-half Trill, one-quarter Bolian, one-quarter Betazoid
 * T'Piam: half-human, half-Vulcan

Star Trek: Zealous

 * Rin: half-human, half-Caitian
 * Ebak Sison: half-human, half-Caitian