If I remember right, the name is mentioned in both Episodes I and II. I’m sure it’s in Episode II, when Darth Tyranus is talking to his master late in the movie. I think it’s mentioned in Episode I, but you really have to be looking for it. And he may be referred to as “Lord Sideous” at some point.
Probably not the most helpful post, but I do remember hearing the name in the movies, though not more than a couple of times.
on page 7 of the star wars novel by george lucas, he states:
“two meters tall. bipedal. flowing black robes trailing from the figure and a face forever masked by a functional if bizarre black metal breath screen–a dark lord of the sith was an awesome, threatening shape as it strode through the corridors of the rebel ship.”
of course in the movie you heard him before you saw him.
You guys had me at ‘rearange the letters’ 
He’s not mentioned by name in Episode I, though if he was it would’ve been by the Neimoidians.
In Episode II Tyranus mentions him by name when taunting Obi Wan in his weird floaty blue-light prison shackles.
Why is that a problem? Would it help if I changed the wording to “it’s pretty clear from the dialogue, as I recall, that the two bad guys, Darth Maul and the character we know as Darth Sidious, but who is not actually named in the movie, are Sith and are basically evil Jedi”?
I thought the word Sith came from the SW novelization, but I wasn’t sure. I knew somebody would, though. Where did the whole “Anakin fell in a pit of lava while fighting Obi Wan” thing come from? I know it was floating around the EU long before it emerged as a plot point in RotS.
That came up in the Return of the Jedi novelisation. But in the same breath they had a different relationship with Uncle Owen explained, so the fans were all very aware that the lava thing was touch and go on whether it would remain canonical.
In Episode I, when they find out that the ambassadors are Jedi, Nute Gunray turns to Rune Haako and says “We must contact Lord Sideous.” They may have used his formal title in other instances, as well.
One of the most confusing things about Episode II for the uninitiated (and this entirely is because of sloppy writing by Lucas) is the way that Count Dooku uses three names (Count Dooku, Darth Tyranus and Lord Tyranus), but never acknowledges any of them but Dooku until the end of the movie, and how he lies constantly about his motivations and actions without there being a great deal to indicate to the audience what precisely he really is doing and why.
Either I wasn’t paying close enough attention, or it was too subtle, because when watching Episode I, I never caught an instance from which I was able to pick up the association between the name “Darth Sidious” and this character. I saw Senator Palpatine without a hood and Senator Palpatine in a hood. In fact, It wasn’t until I looked at the credits that I learned that there was a character called “Darth Sidious” in the movie.
Well, the odd thing about the Star Wars universe is that just about everything is considered canon. What they have is a hierarchy that tells what trumps what when two sources disagree on something.
The movies are at the top of the list. So, if there is a contridiction between another source and the movie, the movie is canon. The novels come next (I think). The various games, toys and comics come somewhere below that.
KOTOR I and II are indeed canon, as are all of the other games, including the role-playing game and the online MMORPG. In their case, the scenarios themselves are canon but the results are not. So, to use KOTOR I for an example (and I’ll use a spoiler box on the off chance that someone hasn’t played it yet)…
The Mandolorian war, the split among the Jedi, Revan and Malak turning to the dark side and so on are canon. The results of the game as you played it are not. (Did Revan return to the Dark Side or redeem himself? Did he take control of the Sith forces and attack the Republic or did he destroy the Sith and save the Republic? For that matter, was Revan a he or a she?)
Left something off. There is a short list of things that specifically are not canon. This list includes commercials and George Lucas in Love of all things, among others. (I think fan fiction falls into this category too.) About half of the Star Wars Christmas Special is included. Unfortunately, the other half isn’t.
Are the damn Ewok movies canon or not? And are they during the Imperial era or after the New Republic was founded?
I’ve held off on this as long as I can (days).
Does anyone else see the title of this thread and think, “Brain, brain. What is brain?”