What is Aikido?
Aikido is a modern form of Budo (traditional warrior arts of Japan) in which primary emphasis is placed on flowing with the momentum and energy of an attack and leading the attacker into any number of techniques designed to send the attacker away or immobilize him (without injury).
The first character is Ai (pronounced "EYE"), and represents Harmony.
The second Kanji is Ki (pronounced "KEY"), representing the vital energy that moves the universe.
The final Kanji is Do (pronounced "DOE"),meaning way, path, or road.
Most martial arts of Japanese descent will either end with "jutsu" (ex. Jujutsu) or "do" (ex. Kendo). A "jutsu" is a series of techniques, but a "do" most often implies a martial art with a higher spiritual or philosophical code that the practitioner must live by, or an art that has been modified to allow for safe competition. A good example would be Judo, in which the deadlier techniques from Jujutsu are eliminated. "Jutsu" arts also have the distinction of being more combat-oriented, where the goal was to kill or disable an attacker.
Therefore, we can understand Aikijutsu (a precursor of, and primary influence for Aikido) as a "Series of Techniques for Harmonizing with Energy," and Aikido as the "Way to Harmonize Energy."
Aikido was developed by a man named Morihei Ueshiba (referred to as O-Sensei, meaning great teacher) primarily during the late 1920's and 30's. O-Sensei studied many martial arts throughout his life. The core martial art that he used in the creation of Aikido was Daito-ryu Aiki-jujutsu, which Ueshiba studied directly from Takeda Sokaku. Along with the empty-handed, throwing and joint-locking techniques of Aikijutsu, O-Sensei incorporated elements from Ken (sword), Jo (short staff), and Tanto (Knife).
Originally O-Sensei called his art "Aiki Budo" (the warriors way of harmonizing energy). He officially named it Aikido in 1942, when the Greater Japan Martial Virtue Society (Dai Nippon Butoku Kai) was engaged in a government sponsored reorganization and centralization of Japanese martial arts following WWII.
Aikido inherits its philosophical influence primarily from a neo-Shinto religion lead by Onisaburo Deguchi, named Omoto-kyo. One of the primary features of Omoto-kyo is its emphasis on the attainment of utopia during one's life. This was a great influence on O-Sensei's martial arts philosophy of extending love and compassion, especially to those who seek to harm others. Aikido demonstrates this philosophy in its emphasis upon mastering martial arts so that one may receive an attack and harmlessly redirect it. In an ideal resolution, not only is the receiver unharmed, but so is the attacker.
Aikido first made it's way to the United States in the 1960's in Hawaii, largely due to Ki Society Founder (and the only 10th degree blackbelt promoted by O-sensei himself) Koichi Tohei. It is through Tohei Sensei that we derive our lineage in the art.
*some info pulled from wikkipedia*