The Three Thousand Worlds is the Roronoa Zoro technique, "Sanzen Sekai" in Japanese, which is one of the strongest among the techniques of his Three Sword Style. Three Sword Style, "Santoryu," is a rather rare method of swordsmanship whereby its user fights with three katanas: one in each hand and the third clenched between the teeth. This unique style is what all the various techniques of Zoro are founded on, one of which is the Three Thousand Worlds.
The attack of the Three Thousand Worlds occupies a special place in Zoro's way as a swordsman. The first time it was shown was during his fight with Dracule Mihawk at the Baratie restaurant. That moment was very critical for Zoro. Although the technique didn't work against the then-best swordsman in the world, it showed how Zoro has grown up and pushed himself beyond the limits.
The name of the technique "Three Thousand Worlds" is derived from Buddhist philosophy. According to the creator, Eiichiro Oda, the inspiration for this technique was taken from Trichiliocosm or "Three Thousand Realms of Existence," linked with Buddhism. This concept describes that in one thought, there can exist three thousand worlds to express the immensity and interrelation of existence, referring to Zoro cutting anything in as many realms using his ability with a sword.
Every time Zoro uses this Three Thousand Worlds, he usually says a very powerful oath: "Nine mountains, eight seas, there is nothing I cannot slice. Three thousand worlds!" This statement not only works as a battle cry but also epitomizes the philosophy of the technique and Zoro's belief in his swordsmanship.
The Three Thousand Worlds attack is, therefore, one of Zoro's uniquely major awesome attacks that was set a number of times throughout the very long-running One Piece series. Its recurring use tells of its importance to Zoro's arsenal and how it turns out to be useful against strong opponents. This technique has become adored by the fans due to the depiction of power and visual effect of the anime, hence making it added into the many forms of media and merchandise of One Piece.
This technique appears as a card in the "Pillars of Strength" set, which attests to the relevance of this franchise in the One Piece Card Game. In a similar sense, action figures that represent Zoro usually have three swords attached to allow collectors to form this famous technique. These representations of the technique in other media formats further indicate its status as an icon within the One Piece universe.
It is important to note that the Three Thousand Worlds is actually a Santoryu technique. It therefore means that Zoro has to be holding all his three swords to attack, since it is different from his One Sword Style or Two Sword Style techniques. How he is to hold three blades simultaneously for such a mighty strike further portrays great skill and control Zoro had as a swordsman.
From the series alone, mastering the technique of Three Thousand Worlds by Zoro has grown parallel to his overall abilities. Though it did not work earlier on Mihawk, with continued training and experiences, Zoro has no doubt harnessed the might and accuracy of this attack. This parallels the character development of Zoro and his unyielding goal to be a great swordsman in this world.
Within the One Piece power scaling, the Three Thousand Worlds is integral to Zoro. It's a technique that signifies his progress from first using it against Mihawk but failing to later using it against stronger and stronger enemies. The progress of this technique is identical to Zoro's path: what he wants to do and achieve after his hard training.
The Three Thousand Worlds technique has also been adapted into video games based on the One Piece franchise so that gamers can use it for themselves. This inclusion in interactive media further cements the status as one of Zoro's most recognizable and potent abilities.