Aikido is a martial art, in which an attack is not countered with an equal physical force. Instead, the force of attack is re-directed to work against the attacker. In an ideal situation one creates circumstances in which an attack does not take place at all.
Koichi Tohei formulated five principles for executing aikido techniques:
1) Ki is extending
2) Know your opponent’s mind
3) Respect your opponent’s Ki
4) Put yourself in your opponent’s place
5) Perform with confidence.
If you rely on physical strength only, you will not be able to deal with an opponent who is stronger than you. That is why much attention is given to leading the mind of the attacker. Obviously, you have to know your own mind first before leading others.
Moreover, these principles are not limited to aikido. One might as well practice ki-judo, ki-football, ki-business management, ki-cooking, or anything else that combines physical activity with the use of the mind. It is just because of historical curiosity that unification of mind and body came to be practiced through aikido techniques.
Aikido techniques can be roughly divided in control techniques (katame-waza), much in their original aiki-jujutsu forms, throwing techniques (nage-waza) which are based on control techniques, and projections (kokyunage) which are the most particular in aikido.
The following list comprises the most essential set of technical principles of aikido.
These are not individual techniques only, but technical principles each having numerous different versions and manifestations.
"From one thing, know ten thousand things" (Miyamoto Musashi in Go Rin no Shō c. 1643).
More detailed descriptions are behind these links:
The word "aikido" is comprised of three characters:
合 – ai – join, unify, harmonize
氣 – ki – energy, spirit, force
道 – dō – way, path.
The last character, dō, refers to a way of life, and can be found in many martial arts (e.g. judō, kendō) and other arts (e.g. chadō - tea ceremony, shodō - calligraphy).
The character for ai is used in verbs "to match" and "to fit", and in compound words such as e.g. "union" and "synthesis".
Characters for ai and ki do not appear together outside martial arts, which has lead to many possible interpretations of the term, one popular being "harmonizing energy". This has a rather esoteric tone, even though it would not need to have.
If you define ki as a whole consisting of mind and body, then aiki simply refers to unification of that whole, and aikidō to the path to that unification, hence "the way of mind and body unification".
The word "ki-aikidō" ultimately states the same thing twice, which is unnecessary for the concept. It underlines the importance of Ki in this style of aikido, but is there, however, mainly for historical and political reasons.