The Transformers was the original Transformers toy line that produced from 1984 to 1991. The series was simply called The Transformers until the relaunch in 1992 was titled Transformers: Generation 2, after which fans applied the name Generation 1 (later made official by Hasbro) as a method of separating the different Transformers eras. Often the word "the" is omitted from the name.

Source: Wikipedia