Chidori (One Thousand Birds) is an A-rank ninjutsu created by Hatake Kakashi and used by Uchiha Sasuke. It is also known as the S-rank ninjutsu, Raikiri (Lightning Cutter).

In reality, they are the same ninjutsu.

The Chidori is a one-hit kill ninjutsu which focuses a large quantity of chakra into the user’s dominant hand who then charges at the target to deliver a high-speed thrust. It requires the user to perform nature transformation to create lightning chakra, and then shape transformation to make it discharge with the range and power they need. It is named after the unique chirping noise that it makes.

Kakashi invented it when he was 13 years old and was his sole original technique. After Kakashi learned the Rasengan, the pinnacle of shape transformation, he tried to combine it with nature transformation as his teacher had intended but found it too difficult. In reaction to this failure, he instead created the Chidori which also combines nature and shape transformation but wasn’t nearly as difficult.

The drawback of the technique was that when used as a high-speed thrust, Kakashi moves too fast to see and react to an enemy’s counterattack. It could still be used this way but not without putting him in danger which made it a dangerous, imperfect technique. However once he acquired Obito’s Sharingan, he was able to perfect it using its Eye of Insight which allowed him to perceive counterattacks and avoid them.

At some point during Kakashi’s career, he used Chidori to cut lightning and it gained the nickname of Raikiri.

Even though it is technically called Chidori, this deed made it famous as Raikiri instead and Kakashi himself decided to use the nickname for the technique as well.

The ninjutsu itself, however, is unchanged. They both use the exact same hand-seal sequence, he can only use them four times in total in Part 1, he describes them interchangeably, and they have the same effect and appearance. It is simply that when wielded by Kakashi, it is known as the S-rank Raikiri instead of the A-rank Chidori it truly is because of its reputation and the feat of splitting a lightning bolt itself.

The story of Chidori/Raikiri is based on the story of Tachibana Dōsetsu, a Japanese samurai:

Dōsetsu was in possession of a famous sword called Chidori. One day, while he was still a young man, he was taking shelter under a tree, as it was raining. Suddenly, a bolt of lightning struck him. However, Dōsetsu used his Chidori to cut the Thunder God inside the lightning bolt, allowing him to survive. After this incident, he renamed his Chidori to Raikiri.

Like the ninjutsu, the sword itself was unchanged but gained a new name.