Wookieepedia - Star Wars:
Sith - Behind the scenes:
An issue of Star-Lord, published by Marvel Comics in 1973, featured a villain, Rruothk'ar, who was described as a "Sith-Lord".
As far as SW canon: BlueHarvest.net Scoops!
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Star Wars:
A New Hope script: Star Wars: A New Hope script STAR WARS - Episode IV.
A NEW HOPE
From the JOURNAL OF THE WHILLS by George Lucas Revised Fourth Draft January 15, 1976 LUCASFILM LTD. ... INTERIOR: REBEL BLOCKADE RUNNER -- MAIN HALLWAY.
The awesome, seven-foot-tall Dark Lord of the Sith makes hisway into the blinding light of the main passageway.
scifi.stackexchange.com/users/976/DVK-on-Ahch-To
I remember Darth Vader being referred to as a "Dark Lord of the Sith" before the prequel trilogy came out, probably before Episode V was released.
I don't think the Episode IV movie uses the phrase, but I think the novelization does.
(The novel is canon, right?)
But there was no explanation of what the word "Sith" means.
scifi.stackexchange.com/users/3025/Keith Thompson
I believe Vader was called the Dark Lord of the Sith around the time of the initial movie's release, though I don't think mention of it was made in the film.
I believe there was mention in the original novel, though.
As Keith said, that concept wasn't expanded upon.
scifi.stackexchange.com/users/656/Jeff
The question asks for 'real time'
as to when people would have known the term 'sith' DVK mentions the ANH scripts , but as far as I can tell these were not (officially) available until around the time of the special edition releases in 1997.
However, the official novelisation Star_Wars: From the Adventures of Luke Skywalker published November 12, 1976, does mention the Sith.
(and notably Palpatine's name for the first time)
Wikipedia notes:
The novel also refers to Darth Vader as a Sith Lord.
Although he was referenced as such in various merchandising tie-ins at the time of the original film , he is not referred to as a Sith Lord in the movie.
In fact, the term Sith Lord is not mentioned in the films until 1999's Star Wars:
Episode I –
The Phantom Menace
I've looked through the star wars vault but
the earliest mention I could see of Sith was on a lolly wrapper from 1980.
So I think the earliest 'canon' mention was in the novelisation in November 1976, but there would have been some unknown PR or merchandising that mentioned it before then possibly before the original film was even released.
scifi.stackexchange.com/users/1132/Jeremy French
Sith were in the initial scripts for the original Star Wars, as a honorable (tho dark) knightly order opposed to the Jedi (who were, at that time, called "Jedi Bendu", IIRC).
This was from before the movies were even made.
Their visible representative was a warrior called Prince Valorum (later, the unused name was recycled by Lucas for the chancellor of the Republic) who assisted the New Imperial general Darth Vader (yes, DV was the name of a repugnant, honor-less general, originally) to deal w/the protagonists of that early draft, tho Vader's vile behavior convinced him to join the heroes, instead.
scifi.stackexchange.com/users/46659/Amra
I first heard the term "Dark Lord of the Sith" when I was like six or seven.
I saw Episode 4 in the Drive-
In theater.
Yeah
I just dated myself - LOL!
Anyway, I was a huge Vader fan after seeing Episode 4 as a kid, and was even him for Halloween 5 straight years before choosing Han Solo because a friend was Chewbacca.
But all things Vader, I had toys, comics and books, story 33 record albums, and trading cards and stickers.
In all the books and even on his original trading card, he is referred to as "The Dark Lord of the Sith".
Also in one of the story books he is referred to as a "Sith warrior".
And as said out here earlier, it's in the novels.
Everything I stated is from the original 70's releases.
However, nobody ever said what the Sith was in any of these original materials.
Oh, I just remembered asking my father what the Sith was, and he said it is probably "Vader's religion".
I guess my Pop was ahead of his time back in 77' - LOL :-)
scifi.stackexchange.com/users/33746/Tony B
I don't remember any mention of the Sith in the original trilogy. The few expanded universe books I've read (which predate the prequel trilogy) refer to "Dark Jedi."  When were the Sith first mentioned in Star Wars canon?  (Real time, not in-universe time.)
When did the Sith enter the Star Wars canon?
scifi.stackexchange.com
<star-wars><sith>
scifi.stackexchange.com/questions/7529
scifi.stackexchange.com/users/2943/Sean McMillan
An issue of Star-Lord, published by Marvel Comics in 1973, featured a villain, Rruothk'ar, who was described as a "Sith-Lord".  However, first "canon" mention of the sith was in the official novelisation, Star_Wars: From the Adventures of Luke Skywalker,  published November 12, 1976, does mention the Sith, but there may have been some unknown PR or merchandising that mentioned it before then, possibly before the original film was even released.   The two earliest mentions of a "Sith" were from a Marvel published comic in 1973 and an official novelisation of Star Wars published in 1976. These seem to be the two earliest references.
The earliest reference to any kind of "Sith" was published by marvel comics in 1973. This  featured a villain, Rruothk'ar, who was described as a "Sith-Lord".   the Adventures of Luke Skywalker  published November 12, 1976, does mention the Sith.