Over the course of his journey, Roronoa Zoro has been in situations where his techniques have had to adapt for one reason or another. In addition to his primary fighting style, he has several other related fighting styles that form the greater part of his Santoryu fighting abilities. Ittoryu (literally meaning "One Sword Style" or "One Blade Style") is one of them.
These are the attacks he uses with one sword. As Zoro is left-handed, his Ittoryu techniques have nearly always been executed with his left hand. He usually uses Wado Ichimonji for them, but after the timeskip he seems to be proficient in using each of his three swords for his one-sword techniques. He mainly fights with one sword when not fighting seriously or when he is using a specialized cutting technique. Early in the series (when Sham stole two of his swords and Zoro had to fight him and Buchi with just one), Zoro himself claims that he is "not that good at one-sword techniques". However, over time, he seems to have worked on this weakness and becomes proficient with this style. In fact, most of his Ittoryu techniques are very powerful finishing techniques.
Before the Timeskip
Ittoryu Iai: Shishi Sonson (一刀流 ｢居合｣ 獅子歌歌,Ittōryū Iai: Shishi Sonson?, literally meaning "One Sword/Blade Style Re-sheath: Lion's Song"): Placing a single, sheathed sword upright and listening to the "breath" of his opponent, Zoro rapidly unsheathes, attacks while passing by his foe, and then resheathes his sword. An interesting note is that Zoro does the technique in reverse grip from beside-and-over his head, as opposed to from the waist with a standard grip on the sword. His very first-seen Ittoryu attack, this move was used by him to finish off Mr. 1. Generally, Zoro prefers using this technique when he needs to cut metal like Mr. 1's steel body or Kaku's Tekkai technique. Pre-timeskip, the attack was as powerful as Kaku's Rankyaku: Rodan. As with many other of Zoro's attacks, this one's name is a pun: "shishisonson" (子子孫孫/子々孫々,"shishisonson"?) is a four-word proverb meaning "one's offspring" or "prosperity", with the "sonson" (歌歌,"sonson"?) of this move deriving from "songu" (ソング,"songu"?), the Japanese way of pronouncing the English "song". In the 4Kids dub, the attack is called Single Sword Lion's Strike, and in the FUNimation dub, it was called One Sword Style: Lion's Strike (but in Episode 299 is renamed to One Sword Style Sword Draw: Lion's Song). Also in the FUNimation dub, "breath" is referred to as "rhythm". In some video games, such as in Jump Super/Ultimate Stars and One Piece: Grand Battle! Swan Colosseum, this is used as a reversal/counter instead of as a direct attack.
Sanjuroku Pound Ho (三十六煩悩鳳,Sanjūroku Pondo Hō?, kanji meaning "Thirty Six Klesha Phoenix"; furigana meaning "Pound Phoenix/Cannon"): This attack uses the air itself to slash the target from a distance. To initiate it, Zoro first holds one of his swords horizontally above the shoulder of his sword arm, and then performs a circular swing that launches the air compressed projectile spiraling towards the target. Zoro introduced this technique as a very powerful Flying Slash Attack. This was first seen being used to finish off Braham in Enel's Survival Game. This technique is known as 36 Pound Phoenix in the VIZ manga, 36 Caliber Phoenix in the FUNimation dub (and sometimes the VIZ manga) and Single Sword Rising Phoenix in the 4Kids dub. The attack's name is actually a very heavy pun: it is written out as "Phoenix of 36 Kleshas" in the manga with a skewed reading attached that makes it "36 Pound Ho" when read out:
Hō (ほう,Hō?) can mean "cannon" or "phoenix'; however the attached kanji is for phoenix (鳳,phoenix?), making that the literal translation and the "cannon" reading a pun on that, both of which are correct. Hō also is the Japanese reading of Phoenix.
Pondo (ポンド,Pondo?) is a skewed reading of bonnō (煩悩,bonnō? "klesha"), a Buddhist term for "worldly desires with evil thoughts", and is a reference to the caliber of a cannon (a 36-caliber cannon would fire a 36-pound ball).
Hiryu: Kaen (飛竜火焔,Hiryū: Kaen?, literally meaning "Flying Dragon: Fire Blaze"): Using one sword wielded in his left hand with his right hand gripping his left wrist for support (or vice-versa), Zoro jumps high up into the air and slashes his opponent. After slashing them, Zoro's opponent then bursts into flames (in the anime, the color of the fire is blue instead) from where they were slashed. This was first seen being used against Ryuma. The animal or creature that usually accompanies Zoro in the background when performing powerful techniques is an occidental dragon. The dragon bares an uncanny resemblance to the dragon killed by Ryuma the King from Oda's earlier work, Monsters. The scene where Zoro slashes Ryuma with this technique also resembles the scene from Monsters, in which Ryuma slays the dragon. In the VIZ manga and FUNimation dub, this is called Flying Dragon Blaze, a translation of the original name. The kanji "kaen" can also be written as "火炎" (lit. fire flame) instead.
During the battle on Onigashima, Zoro used a stronger version of this attack in with Enma. It was strengthen from his own Busoshoku Haki, but also the strange Haki bearing Oden's presence stored in Enma as well, causing a large black flame-like aura which seemed strong enough for Big Mom to warn Kaido to dodge it. Although, Zoro did miss, he cleanly sliced off one of the horns of the Skull Dome. Most notably, unlike the original attack, he his opponent didn't burst into flames after being slashed, it looked as if he released a flaming slash instead meaning it could've also been enhanced through the Kitsunebi-ryu he "stole" from Kin'emon.
After the Timeskip
This is a list showing the attacks Zoro used for his Ittoryu after his two years of training with Dracule Mihawk. The techniques he uses after the two years are mostly enhanced versions of his old ones.
Yakkodori (厄港鳥,Yakkōdori?, literally meaning "Disaster Harbor Bird"): Zoro launches from his sword a crescent moon-like projectile of compressed air to the opponent with great speed, similar to 36 Pound Cannon. "Yakko Odori" is a traditional Japanese dance. It was first seen used against Hody Jones. It was powerful enough to critically injure one of Hody's subordinates whom he used as a shield. In the VIZ manga and FUNimation subs, this is called Bird Dance.
Sanbyakurokuju Pound Ho (三百六十煩悩鳳,Sanbyakurokujū Pondo Hō?, literally meaning "360 Pound Cannon" or "Three-Hundred-Sixty Klesha Phoenix"): Zoro delivers a stronger version of his 36 Pound Cannon, which is presumably 10 times stronger than the original. It was first seen used against a big fish the Straw Hat Pirates caught in their voyage to the New World. In the FUNimation subs, this is called 360-Pound Phoenix. The original naming pun origins apply as with all the variations of this technique.
Ittoryu Iai: Shi Shishi Sonson (一刀流 ｢居合｣ 死・獅子歌歌,Ittōryū Iai: Shi-Shishi Sonson?, literally meaning "One Sword/Blade Style Re-sheath: "Death-Lion's Song""): A stronger version of his original "Ittoryu Iai: Shishi Sonson", where Zoro swings his sword in a very quick and powerful unsheathing move to slice his opponent; the reverse grip is still used, though Zoro no longer draws his sword from beside-and-over his head, and instead draws it normally from one of his hips. This attack is strong enough to easily decapitate a legendary beast like a Dragon. It was first shown to decapitate the Punk Hazard dwelling dragon; the way Zoro does it is mainly through leaping up into the air with either his own strength (or with outside components such as Usopp's assistance), then uses his mouth's sword to slice up his target in a spiraling wind. The version of that said technique was used in midair in that case (where Zoro moves forward a distance through the air). In the VIZ manga and Funimation dub of Film Gold, this is called Iai Death Lion Song but in the FUNimation subs it is called Lion Strike. In the english localization of both One Piece: Pirate Warriors 2 and One Piece: Pirate Warriors 3, it is called Death Lion Song; Zoro performs it in a style akin to how he used it against the aforementioned dragon, only the spiraling leap can even conjure up a Tatsu Maki in those two games' cases (when fully charged up). The only instance so far that it's been used on the ground is in Burning Blood as his weak guard break attack (though Zoro spirals through the air in a drilling motion akin to Hyokindama before returning to grounded-stance to re-sheath as he brakes from the momentum).
Daishinkan (大辰撼,Daishinkan?, literally meaning "Great Dragon Shock"): First used as a finisher move against Monet, Daishinkan consists of Zoro running towards his opponent with Shusui held outstretched in one arm. He then executes a two handed overhead swing that can cut people in half, such as Monet's snow body. The attack resulted in a powerful shockwave that cut through the floor many meters ahead of him. In the Viz manga, this is called Dragon Quake and in the FUNimation subs and dub, this is called Great Dragon Shock.Shin/Tatsu (辰) is a Japanese term for the Dragon zodiac sign, while 撼 literally means to "move"; in both those cases, the technique is most likely via its localization is a pun off of the term shinkan (震撼), which means to "shake".
Baki (馬鬼,Baki?, literally meaning "Horse Ogre"): After raising his sword above his head, Zoro delivers a powerful vertical slice to his opponent, usually at an angle along with not being as exaggerated as Daishinkan. This attack can easily slice through solid stone. It was first used to counter Pica's Bitestone. In the Viz Manga, this is called Demon Horse.
↑One Piece Manga and Anime — Vol. 21Chapter 195 (p. 19) and Episode 119, Zoro first uses Ittoryu against Mr. 1 to defeat him by cutting through his steel body.