Retronym

A retronym is a new name for something that already exists. This is so that people can tell the older thing apart from the newer one.
For example, the name "acoustic guitar" was made to tell them apart from electric guitars. Before electric guitars, acoustic guitars were just called guitars because there were no other kinds of guitars. Therefore, the name "acoustic guitar" is a retronym for the word guitar.
In pop culture, the 1977 movie ' was renamed ' in 1981, one year after the sequel "" came out.