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.

合 (Ai) is mainly used in compounds to mean 'combine, unite, join, meet', examples being 合同 (combined/united), 合成 (composition), 結合 (unite/combine/join), 連合 (union/alliance/association), 統合 (combine/unify), and 合意 (mutual agreement).

Therefore, from a purely literal interpretation, aikido is the "Way of combining forces" or "Way of unifying energy", in which the term aiki refers to the martial arts principle or tactic of blending with an attacker's movements for the purpose of controlling their actions with minimal effort.

[14] Ueshiba envisioned aikido not only as the synthesis of his martial training, but as an expression of his personal philosophy of universal peace and reconciliation.

Additionally, Ueshiba is known to have studied Tenjin Shin'yō-ryū with Tozawa Tokusaburō in Tokyo in 1901, Gotōha Yagyū Shingan-ryū under Nakai Masakatsu in Sakai from 1903 to 1908, and judo with Kiyoichi Takagi (高木 喜代市) (1894–1972) in Tanabe in 1911.

Along with empty-handed throwing and joint-locking techniques, Ueshiba incorporated training movements with weapons, such as those for the spear (yari), short staff (jō), and possibly the bayonet (銃剣, jūken).

[10] After Ueshiba left Hokkaidō in 1919, he met and was profoundly influenced by Onisaburo Deguchi, the spiritual leader of the Ōmoto-kyō religion (a neo-Shinto movement) in Ayabe.

The idea was a great influence on Ueshiba's martial arts philosophy of extending love and compassion especially to those who seek to harm others.

[19] In addition to the effect on his spiritual growth, the connection with Deguchi gave Ueshiba entry to elite political and military circles as a martial artist.

Tohei left as a result of a disagreement with the son of the founder, Kisshomaru Ueshiba, who at that time headed the Aikikai Foundation.

[27] A final major style evolved from Ueshiba's retirement in Iwama, Ibaraki and the teaching methodology of long term student Morihiro Saito.

Today, the major styles of aikido are each run by a separate governing organization, have their own headquarters (本部道場, honbu dōjō) in Japan, and are taught throughout the world.

Ki has many meanings, including "ambience", "mind", "mood", or "intention and action", however, in traditional martial arts and medicine it is often referred to in its more general terminology as "life energy".

Gozo Shioda's Yoshinkan Aikido, considered one of the "hard styles", largely follows Ueshiba's teachings from before World War II, and surmises that the secret to ki lies in timing and the application of the whole body's strength to a single point.

For example, many dōjōs begin each class with warm-up exercises (準備体操, junbi taisō), which may include stretching and ukemi (break falls).

In more advanced training, uke will sometimes apply reversal techniques (返し技, kaeshi-waza) to regain balance and pin or throw tori.

[4] Many of the strikes (打ち, uchi) of aikido resemble cuts from a sword or other grasped object, which indicate its origins in techniques intended for armed combat.

[28] Others consider atemi, especially to the face, to be methods of distraction meant to enable other techniques; a strike, even if it is blocked, can startle the target and break their concentration.

Additionally, the target may also become unbalanced while attempting to avoid a strike (by jerking the head back, for example) which may allow for an easier throw.

[41] Weapons training in aikido traditionally includes the short staff (jō) (these techniques closely resemble the use of the bayonet, or Jūkendō), the wooden sword (bokken), and the knife (tantō).

Some schools, such as the Iwama style of Morihiro Saito, usually spend substantial time practicing with both bokken and jō, under the names of aiki-ken, and aiki-jō, respectively.

Freestyle practice with multiple attackers called randori (乱取) is a key part of most curricula and is required for the higher-level ranks.

[26][45] In applying a technique during training, it is the responsibility of tori to prevent injury to uke by employing a speed and force of application that is appropriate with their partner's proficiency in ukemi.

[46] Aikido training is mental as well as physical, emphasizing the ability to relax the mind and body even under the stress of dangerous situations.

[50] Morihei Ueshiba once remarked that one "must be willing to receive 99% of an opponent's attack and stare death in the face" in order to execute techniques without hesitation.

[51] As a martial art concerned not only with fighting proficiency but with the betterment of daily life, this mental aspect is of key importance to aikido practitioners.

[54] Most aikido systems add a pair of wide pleated black or indigo trousers known as hakama (used also in Naginatajutsu, kendo, and iaido).

Stanley Pranin has observed that some criticism may stem from weak attacks from uke, allowing for a conditioned response from tori, resulting in underdevelopment of the skills needed for the safe and effective practice of both partners.

[55] To counteract that some styles allow students to become less compliant over time, however in keeping with the core philosophies, that is after having demonstrated proficiency in being able to protect themselves and their training partners.

[26] Such adaptations are debated between styles, with some maintaining that there is no need to adjust their methods because either the criticisms are unjustified, or that they are not training for self-defense or combat effectiveness, but spiritual, fitness or other reasons.