The term "original series" refers to a group or collection of items that were all created at the same time, and are considered the original version or iteration of something. It can also refer to a television show or movie that is part of a larger franchise but was released first in its original form, before any sequels, prequels, remakes or adaptations. In other words, it's the "original" series because it came out first and set the stage for all subsequent iterations.