Cy is the nickname Starbuck gives to a Cylon Centurion from the Galactica 1980 episode "The Return of Starbuck". As it turns out, at the end of the episode, when three new Cylons arrive to rescue him, Cy reveals that his full designation is "Cyrus."
Cy is a Cylon centurion rebuilt from three crashed centurions and a crashed Cylon Raider to provide Lieutenant Starbuck a companion on the planet on which he and the Cylons crashed. Cy expressed an amazing capability to learn and to participate in conversation, even about issues that he expressed consternation over, such as why he had to continue operating the heat generator when it was the pregnant Angela's turn, or as to why Humans and Cylons are enemies, or as to why Starbuck's playing of a game of Pyramid contradicts the stated rules.
Cy also demonstrated a thorough depth of thought process when Starbuck asked him what the Cylon empire intended to do with the universe once they had organized it (a mission statement that Cy himself stated was as being the official Cylon goal). Cy thought over this question, and mentioned to Starbuck that he didn't know what would happen, and that no one had ever asked him that question. Cy eventually dies, sacrificing his life for Starbuck's life against fellow Centurions who land upon the planet and who would've killed Starbuck, Angela, and the baby had they discovered them.
Gary Owens provided the primary voice-over for Cy, complete with vocal inflections. Barely audible underneath this voice-over, however, is the generic, monotone Cylon voice used throughout the original Galactica series.