It was about 1942 that Morihei Ueshiba began to use the term "Aikido" for the system of martial art techniques he was teaching. Until then he had called it "Aikibudo" (budo = 'the martial way'), which is the Japanese term used for martial arts in general or "Aikinomichi" (no michi = 'the way of').
As Ueshiba got older, his martial outlook mellowed in no small part due to his involvement with the religious cult of Omoto. In his later years he endeavoured to achieve harmony as a spiritual concept and looked to how he could also apply it to the way he looked at martial arts. Therefore the emphasis of his martial art moved from a strictly martial focus (killing or incapacitating an attacker) to developing a form that practitioners might use to defend themselves while at the same time also reducing serious damage to an attacker.
Technically Aikido has roots in several styles of Jujutsu (from which modern Judo and Brazilian Ju-jitsu are also derived), but it owes a greater debt to Daitoryu-Aiki-Jujutsu, as well as some sword and spear fighting arts. Many movements in Aikido represent the empty handed equivalent of sword techniques, which becomes especially obvious in Tegatana dosa exercise.
Aikido makes use of body movement (tai sabaki) to blend with an attacker. For example, an "entering" (irimi) technique consists of movements inward towards the attacker, while a "turning" (tenkan) technique uses a pivoting or circular motion around an attacker. Additionally, an "inside" (uchi) technique takes place facing an attacker, whereas an "outside" (soto) technique takes place to the side; a "front" (omote) technique is applied with a forward motion and a "rear" (ura) version is applied with backwards movement, usually by incorporating a turning or pivoting motion.
From a very limited number of basic principles there are numerous possible variations, implementations and combinations possible. For instance, ikkyō or ōshi-taoishi can be applied to an opponent moving forward with a strike, or to an opponent who has already struck and is now moving back to re-establish distance.
As a result, over time, everyone practicing Aikido, irrespective of style, school or organisation, will develop his or her own way based on body stature and personal preferences. One of the insights of Ueshiba was to encourage development and he is recorded expressly encouraging his master students to seek and define their own way of Aikido for the benefit of the art.
looking for a Martial Art with a difference? Try Shodokan / Tomiki Aikido
Different styles of Aikido
Daitō-ryū Aiki-jūjutsu (大東流合気柔術)
Notes on O'Sensei