Galvatron is the name of several fictional Transformers, most often the recreated version of Megatron, the Decepticon leader. He was voiced by Leonard Nimoy in the 1986 Transformers movie, and then by Frank Welker in season 3 and 4 of the animated television series. Since then, other Transformers characters have shared the name Galvatron. Most unique among the characters named Galvatron was the main villain of the anime series Beast Wars Second, who was not a former Megatron, but instead an unrelated Predacon who named himself after the original Galvatron.
Read more about Galvatron: Transformers: Generation 1, Knowledge of Galvatron's Past in G1 Animated Continuity, Beast Wars II, Transformers: Robots in Disguise, Unicron Trilogy, Transformers: Timelines (Shattered Glass)