Aikido is a Japanese martial art founded by Ueshiba Morihei O-Sensei. One way to define Aikido is by comparing the meanings of the different martial arts' names. Tae Kwon Do means "Kicking and Smashing", which is what you are taught to do when confronted. Karate means "Empty Hand" -- a hand which is formed into a variety of weapons to destroy the enemy. Aikido, on the other hand, means "The Way of Harmony". It is Aikido's philosophy of peaceful resolution when contending with violence that sets it apart from the other arts. Aikido aims to achieve victory in a Martial Art Way without having to fight.
To learn more about O Sensei and the history of Aikido, follow this link to the excellent book "Abundant Peace" by John Stevens.