Edmund H. North, who wrote The Day the Earth Stood Still, also created the alien language used in the film, including the phrase "Klaatu barada nikto". The official spelling for the phrase comes directly from the script (as shown in the above image) and provides insight as to its proper pronunciation.
The phrase was never translated in the film and neither Edmund North nor 20th Century Fox ever released an official translation.
Toward the end of the film, Klaatu is pursued by the American military and orders Helen Benson to memorize the phrase, saying "There's no limit to what he [Gort] can do. He could destroy the Earth... If anything should happen to me you must go to Gort, you must say these words, 'Klaatu barada nikto', please repeat that.".
Shortly after, Klaatu is shot and killed; and Gort, knowing he has died, dissolves the polymer cube that encases him, and disintegrates the two guards standing watch. Helen Benson watches this but conveys Klaatu's message. Gort then takes her inside the spaceship and retrieves Klaatu's lifeless body, which he revives.
Usage in 2008 remake
In the 2008 remake of The Day the Earth Stood Still, this line was added at Keanu Reeves' insistence;  wherefore Klaatu uses it near the beginning of the movie to shut down Gort, and again (highly distorted and barely audible) at the end when he stops the destruction of the Earth. Although the line is not in the English subtitles, it can be heard.
Because there is no official translation of the phrase, a few notable attempts have been made to determine the phrase's meaning:
Philosophy professor Aeon J. Skoble speculates the famous phrase is part of a fail-safe feature used during diplomatic missions, whereby Gort's deadly force can be deactivated in the event the robot is undesirably triggered into a defensive posture. Skoble observes that the theme has evolved into a "staple of science fiction that the machines charged with protecting us from ourselves will misuse or abuse their power." In this interpretation the phrase apparently tells Gort that Klaatu considers escalation unnecessary.
Fantastic Films magazine explored the meaning of "Klaatu barada nikto" in a 1978 article titled The Language of Klaatu. The article, written by Tauna Le Marbe, who is listed as their "Alien Linguistics Editor," attempts to translate all the alien words Klaatu used throughout the film. In the article, the literal translation was "Stop Barbarism (I have) death, bind" and the free translation was "I die, repair me, do not retaliate."
The documentary Decoding "Klaatu Barada Nikto": Science Fiction as Metaphor examined the phrase "Klaatu barada nikto" with some of the people involved with The Day the Earth Stood Still. Robert Wise, director of the film, related a story he had with Edmund North saying North told him, "Well, it's just something I kind of cooked up. I thought it sounded good."
Billy Gray, who played Bobby Benson in the film, said that "barada nikto must mean... save earth". Florence Blaustein, widow of the producerJulian Blaustein, said North had to pass a street called Baroda every day going to work and said, "I think that's how that was born."Film historian Steven Jay Rubin, recalled an interview he had with North when he asked the question, "What is the direct translation of Klaatu barada nikto, and Edmund North said to me 'There's hope for earth, if the scientists can be reached.'"
In popular culture
"Klaatu barada nikto" has been used extensively in popular culture. The Robot Hall of Fame describes the phrase as "one of the most famous commands in science fiction" and Frederick S. Clarke of Cinefantastique called it "the most famous phrase ever spoken by an extraterrestrial."
In the film Army of Darkness of the Evil Dead series Ash has to speak similar words in order to retrieve the Necronomicon but fails to remember it properly ("Klaatu... verata... n... Necktie. Nectar. Nickel. Noodle."). In the end he speaks the words again after consuming a potion, allowing him to return to his era. The words were altered from their original use because their meaning was unclear in the original.
The phrase has also been used humorously, as is the case when Senator Alan K. Simpson's office used it. The June 7, 1994, edition of the now-defunct supermarket tabloid Weekly World News reported that 12 U. S. Senators were aliens from other planets, including Simpson. Then-Senator Simpson's spokesman Charles Pelkey, when asked about Simpson's galactic origins, told the Associated Press: "We've got only one thing to say: Klaatu barada nikto."
The phrase was used in an off-hand fashion by Commander John Crichton in the TV series Farscape, in the season 4 episode "I Shrink Therefore I Am".
Protagonist Charlie Baker speaks the words in the play The Foreigner by Larry Shue (who misspells Klaatu's name). At first Charlie, a meek proofreader for a science fiction magazine, tells a friend, "I sometimes wonder whether a science-fiction magazine even needs a proofreader. Does anyone really care whether there is one K or two in "Klaatu, barada, nikto"? Later, he uses the phrase as part of a tirade against a superstitious Ku Klux Klansman who is threatening to lynch him:
"You—dare—to—affront—me? I, who have lain in wait, lo, these many centuries for such a night as this! ... I, child of Hrothgar and of Moloch! I, whom the Old Ones have given suck, to rise now from the forest mold and smite thee! Klaatu! Barada! Nikto! ... You dare to sneer at me! You—puny earthling!"
In the Firefox web browser, the about:robots page is an Easter egg containing a small number of robot references, with the page title reading Gort! Klaatu barada nikto!
In the Armor Games flash game "Kingdom Rush", when you upgrade the magical defense tower from an "Adept Tower" to a "Wizard Tower" it shouts out the phrase as a battle cry.
In the 1982 movie TRON, Alan Bradley's cubicle has a sign that reads "Gort, Klaatu Barada Nikto."
In the video game Minecraft, one can construct an enchanting table used to upgrade items, tools and weapons. The spells listed in the enchanting table's book are, in another pop culture reference, written in the Commander Keen Standard Galactic Alphabet. One can decode the English cipher the spells contain to find many hidden words, with three of the hidden words being "klaatu", "barada", and "niktu", with "nikto" being misspelled.