Cylons are a fictional artificially-intelligent "species" envisioned in the Battlestar Galactica science fiction series and related franchises. Originally created to serve human needs like other machines, a series of events involving the transfer of a human's consciousness into a Cylon's neural network led Cylons to evolve into sentient, self-aware beings. The original Cylons were purely mechanical.
The Cylon was reimagined in the second Battlestar Galactica television series and its prequel Caprica. The first configurations (Centurions) were robots. Others were cyborgs whose different configurations incorporated biological elements, including some that were nearly indistinguishable from humans.
One of the series’ core themes is ‘All of this has happened before and will happen again’. Cylon Centurions were invented twice: Once thousands of years before the events of Battlestar Galactica and later they fought a war against the Thirteenth Tribe on Earth.
The second Centurion creation is detailed in the prequel series Caprica. There they were created by Zoe Graystone and her father Daniel Graystone, owner of Graystone Industries.
The Caprica Cylon is a combination of programming done by Zoe and Daniel's MCP-enhanced ("Meta-Cognitive Processor") Graystone Industries U-87 Robot. A common activity on Caprica was to participate in a virtual world (V-World) that allowed players to live an alternate existence. The original Cylon was developed as a place to house Zoe's avatar (virtual existence) after she is killed in a bombing on a maglev train in Caprica Episode 1. Her father sees her avatar in V-World and forces her best friend Lacy to take him to her there where she explains that she is much more than a simple digital simulation. She has Zoe's memories from a neural scan Zoe developed to extend a V-World character into a virtual instance of the person. The digital Zoe says to her father, "I don't feel like a copy", and she becomes the basis of the first Cylons.