The term "Combiner” refers to a sub-group of Transformers able to combine their bodies and minds into a singular, larger, more powerful super-robot. The term "Combiner" comes from the sub-groups' ability to "combine" into a larger robot. The first combiner was G1 Devastator, formed from the Constructicons, but the technology was eventually duplicated by the Autobots, leading to the creation of more combiner teams by both sides.
Combiner technology has its downsides as well; the primary one being that the combined robot can only do what all of its components agree upon. For beings such as Devastator, that is typically wanton destruction and nothing else. There is a flipside to this: Computron's main problem is that every problem must be worked out to every conceivable solution; even with Computron's enhanced computers, this is a time-consuming task, which for example led to his quick defeat by Abominus (another Decepticon combiner) around the time of the Hate Plague.
Combiners aren't used in Transformers Legends
Combiner sub groups: