Future Goku (未来の悟空, Mirai no Gokū; lit. "Goku of the Future") is the alternate timeline counterpart of Goku that appeared in the timeline in which Future Trunks and Future Gohan lived. This version of Goku makes his only appearance on his deathbed in the brief opening scene of Dragon Ball Z: The History of Trunks, and in a flashback during an episode of Dragon Ball Z.
This version of Goku is far weaker than his mainstream counterpart, because he had not achieved any forms beyond the un-mastered Super Saiyan due to the fact that he died six months before the Androids arrived and murdered all of the Z Fighters, although it is possible that he could have trained in the other world and achieved higher Super Saiyan transformations.
Future Goku lives the same life as Present Goku until his return to Earth after defeating Frieza on Planet Namek. In the his timeline, Future Goku uses the Instant Transmission technique to return to Earth, where he successfully kills Frieza and King Cold when they arrive in August of Age 764.
Future Goku contracts an incurable heart virus in Age 766 and dies six months before the appearance of Future Android 17 and Future Android 18; which leaves the Z Fighters without their strongest warrior. The androids kill the Z Fighters in May of Age 767, except for Future Gohan (Future Goku's son), and Future Trunks. Future Bulma is determined that Goku would have defeated the androids, had he been given the opportunity, so she sends her son back in a time machine to solicit Goku's help.
Future Goku is 29 years old when he dies, and he could not be wished back with the Dragon Balls because he died of a natural cause.