Synth is the term for a class of alien creatures turned into war machines, and one of the several species that make up the
Combine organization. They are officially described as "self-replicating robots that evolve", "created or taken over by the Combine and enslaved during their conquests and wars." Most Synths seem to possess both organic and robotic characteristics, suggesting that they may be the cyborg derivatives of alien races that were previously enslaved by the Combine and assimilated into its armed forces.
