Literally, Aikido is made up of three Japanese words: Ai (harmony), Ki (energy), Do (way). It is often translated as "the Way of harmony with universal energy." Aikido was developed by Ueshiba Morihei-sensei in the 1920s and was given its current name in 1942. However, the roots of Aikido can be traced back to the samurai techniques of feudal Japan, particularly hand, sword, and staff movements. Ueshiba-sensei took this very martial background and changed it into a way to study harmony and cooperation. Read more about the history of Aikido.
Aikido is often classified as a martial art, but this is not technically correct. In Aikido we do not study how to fight or oppose each other. Instead, we study how to fit with each other and create harmony. An Aikido technique requires two people: one to lead and one to follow. The leader practices being in balance and providing strong, clear movement. The follower practices being off-balance and completely giving themselves to their partner's movements. In class we study both parts of Aikido, learning to throw and to be thrown; to pin and to be pinned. Both can be challenging studies and both are needed to make a full and harmonious Aikido technique.
To answer a common question, yes, Aikido can be used in a martial way, where it can be very effective. It is learned by numerous groups including the Japanese riot control police corps. However, it is important to remember that this is not the focus of our training. Thinking of Aikido in this martial way can, in fact, be counterproductive since it can make the relationship between partners adversarial rather than cooperative. Harmony, or fit, is central to Aikido and can be lost if students focus on force and control. As such, we train with the idea that a technique is a cooperative effort between a leader and a follower.