An Omnic is a type of robot with artificial intelligence. Omnics were originally designed and built by humans, mostly notably the Omnica Corporation, to serve the economy.
During the Omnic Crisis, Omniums started producing omnics with weaponry and military strategy to fight humans alongside various god programs that would hack and control local omnics to do their bidding. They were eventually stopped and shut down by multiple military forces such as Overwatch.
Since then, omnics have been second-class citizens in the majority of the world. For example, in London, the omnics have almost no rights, live underground, and have to deal with constant anti-omnic terror attacks. But there are a few exceptions, such as in Numbani, where the omnics are treated equally.