text: Although this TV series had the title of Star Trek , it has acquired the retronym of Star Trek : The Original Series to distinguish it from the numerous sequels that have followed it , and also from the fictional universe that it created .
simplified: Though the original series was just called Star Trek , it got the nickname Star Trek : The Original Series ( ST : TOS or TOS ) to make it different from the spinoffs , and from the Star Trek universe or franchise they take up .