A Powermaster is a Transformers toy where a smaller robot/Nebulan in body armor transforms into an external engine for the vehicle mode of the larger robot, and which needs to be connected to the vehicle in order to unlock the transformation to robot.
The Powermaster Transformers never appeared in the cartoon, having been introduced after the last episode aired. In the Transformers comics the Powermasters were created when a pair of Decepticon jets arrive on Nebulos (where the Targetmasters and Headmasters were created) and repowered themselves using poisonous fuel. The Decepticons allied themselves with two Nebulans who stole a technology to turn themselves into the power source for machines and became the Powermaster engines for the Decepticons.
When four Autobots arrived shortly after carrying a disk with Optimus Prime's personality on it, hoping to build him a new body, they found out about the Decepticons who were terrorizing the planet. In fighting to stop them, three of them were weakened and nearly out of fuel themselves. They, along with the newly-completed Prime, needed to be given the poisoned fuel in order to survive and thus volunteered for the Powermaster process themselves. The newly strengthened Autobots drove the Decepticons from Nebulos and then left themselves.
Compare to Targetmaster and Headmaster. | 
