SkyNet

Skynet is a fictional artificial neural network-based conscious group mind and artificial general superintelligence system that serves as the antagonistic force of the Terminator franchise.

In the first film, it is stated that Skynet was created by Cyberdyne Systems for SAC-NORAD. When Skynet gained self-awareness, humans tried to deactivate it, prompting it to retaliate with a nuclear attack, an event which humankind in (or from) the future refers to as Judgment Day. John Connor forms a human resistance against Skynet's machines in the future, which include Terminators, and ultimately leads the resistance to victory. Throughout the film series, Skynet sends various Terminator models back in time to attempt to kill Connor and ensure Skynet's victory.

The system is rarely depicted visually in any of the Terminator media, since it is an artificial intelligence system. In Terminator Salvation, Skynet made its first onscreen appearance on a monitor primarily portrayed by English actress Helena Bonham Carter and other actors. Its physical manifestation is played by English actor Matt Smith in Terminator Genisys. In addition, actors Ian Etheridge, Nolan Gross and Seth Meriwether portrayed holographic variations of Skynet with Smith.

In Terminator: Dark Fate, which takes place in a different timeline to Terminator 3: Rise of the Machines and Terminator Genisys, Skynet has been erased from existence after the events of Terminator 2: Judgment Day, and another AI, Legion, has taken its place. In response, Daniella Ramos forms the human resistance against Legion, which prompts the AI to attempt to terminate her from the past as Skynet tried with John Connor.

Source: WikiPedia