Yes, Sith was a term that was almost always (if not always) part of the Star Wars lexicon. It just didn't appear in the movies. Padawan... is a term that I really can't stand and wish hadn't been introduced to said lexicon. Apprentice just sounds so much cooler and more serious. It's not an original word, but it works. Padawan just sounds like some amphibious creature related to a frog. Younglings is even worse. Death to that word!

But, ahem, back on topic. Darth Vader was always known as "Dark Lord of the Sith." Padawan was just something that popped up in the prequels. Younglings was only okay when Yoda said it because I figured it was some sort of Yoda speech, but when other characters started saying it... bad things happened.