Aikido was developed between the 1920's to the 1940's by Morihei Ueshiba, known as O Sensei (Great Teacher). The word Aikido is formed by the 3 kanji seen above. "Ai" means to unify, or combine. "Ki" denotes energy, or spirit. "Do" is the path, or the way. When combined, Aikido is translated as the "Way of Harmony with the Spirit", or "The Way of Combining Energy". This is the foundation of the philosophy behind Aikido. In an idealized usage of Aikido neither you nor your opponent are seriously injured. It is this high ideal that makes Aikido a lifelong pursuit.
At its technical root Aikido is a combination of Daito-ryu Aiki-jujutsu, other jujutsu forms, kenjutsu, and jojutsu with a strong philosophical, even moral, code. Ueshiba saw his martial art as a way of promoting peace. It follows then that the throws, joint locks, and pins originally developed to incapacitate, permanently disable, and seriously injure were altered in slight, but significant ways. This allows us to practice Aikido at full speed, and intensity. It is a unique martial art because it is performed by blending with the motion of the attacking person and redirecting the force of the attack rather than challenging it head-on. Entering and turning movements are used as techniques are completed with different combinations of throws, pins and joint locks.
Aikido has spread throughout the world today under the Aikikai Foundation. After O Sensei's passing in 1969, his son Kisshomaru Ueshiba was installed as Doshu (Master of the Way). Eventually, he came to be referred to as Nidai (2nd) Doshu, and it was he who was responsible for codifying the technical syllabus and organizing the Foundation. Presently, Moriteru Ueshiba, grandson of O Sensei, has assumed the position of Sandai (3rd) Doshu. Doshu is the official leader of the world Aikikai Foundation.
From the Aikikai website, "The Aikikai Foundation, officially recognized by the Japanese government in 1940, was founded in order to preserve and promote the ideals of the true Aikido created by the Founder. As the Aikido World Headquarters, it is the parent organization for the development and expansion of Aikido throughout the world."