In the Terminator film series, a terminator is an autonomous robot, typically humanoid, originally conceived as a virtually indestructible soldier, infiltrator and assassin.
James Cameron introduced the first terminator character in the 1984 film The Terminator, featuring a single android simply called "The Terminator", portrayed by Arnold Schwarzenegger. When later Terminator films introduced additional models, some sources retroactively gave Schwarzenegger's character a model number, leading to multiple conflicting names.
In the Terminator universe, a terminator is a formidable "cybernetic organism", robotic assassin and soldier, designed by the military supercomputer Skynet for infiltration and combat duty, towards the ultimate goal of exterminating the human resistance. Terminators are indistinguishable from humans, can speak naturally, copy the voices of others, read human handwriting, and even sweat, smell, and bleed. Because dogs are able to smell the difference, the human resistance uses dogs to help spot them with German Shepherds being the favorite breed. In addition, Terminators can detect their fellow androids and other types of Skynet-related units when they are nearby, as demonstrated in the film Terminator 2: Judgment Day and Terminator Genisys.
A trait persistent throughout the series is the faint red glow of the "robotic eyes" when online, which dim to nothing when a terminator shuts down. In all five movies, the lack of the glow has been used to show when one is out of action. The trait is so characteristic that light-up eyes are often found on Terminator merchandise, with some even replicating the dimming/reillumination effect that occurs during shut down or start up.