Aikido is a Japanese martial art developed by Morihei Ueshiba as a synthesis of his martial studies, philosophy, and religious beliefs. Aikido is often translated as "the Way of unifying (with) life energy" or as "the Way of harmonious spirit."

Ueshiba's goal was to create an art that practitioners could use to defend themselves while also protecting their attacker from injury.

Aikido is performed by blending with the motion of the attacker and redirecting the force of the attack rather than opposing it head-on. The aikidoka (Aikido practitioner) "leads" the attacker's momentum using entering and turning movements. The techniques are completed with various throws or joint locks.  

Aikido derives mainly from the martial art of Daitō-ryū Aiki-jūjutsu, but began to diverge from it in the late 1920s, partly due to Ueshiba's involvement with the Ōmoto-kyō religion.  Ueshiba's early students' documents bear the term aiki-jūjutsu. Many of Ueshiba's senior students have different approaches to aikido, depending on when they studied with him. Today Aikido is found all over the world in a number of styles, with broad ranges of interpretation and emphasis. However, they all share techniques learned from Ueshiba and most have concern for the well-being of the attacker.

Find out about modern day Aikido here.
 

Wikipedia has an excellent article on the history of Aikido (from which the information was taken).  Please click here to read more of the article.