Aikido
Founded
in 1941 (before that known as Aikibudo, and Aikibujutsu) by Morihei Ueshiba,
Aikido is the synthesis of techniques from jujutsu, kenjutsu, sojutsu (spear
fighting), jukendo (bayonet fighting) and O'sensei Morihei Ueshiba's
insightful philosophy. Aikido emphasizes the principle of harmonizing with
an opponents force.
O'sensei
Morihei Ueshiba's first studied Kito Ryu Jujutsu under Tokusaburo Tozawa1. He then studied Tenjin Shinyo Ryu Jujutsu under Takisaburo Tobari and also attended a Shinkage
Ryu Kenjutsu school at this time2. O'sensei's next major martial arts
experience was the Jukendo (bayanet)
training in the Japanese army. Also during his army
years, O'sensei
studied Goto Ha Yagyu Shingan Ryu
Jujutsu under Masakatsu Nakai, which also 
included the use of the sword,
spear and jo. In 1908, after 5 years of studying, O'sensei was awarded a
teaching license from Nakai3. The final significant martial arts experience of O'sensei's life was his study of
Daito Ryu Jujutsu under its Master Sokaku Takeda4. In 1915 after only 30 days of
training, day and night, O'sensei was awarded his first level teaching
license by Takeda. In 1919 he was awarded a complete teaching license,
having mastered Daito Ryu. There are also a few other notable arts studied
by O'sensei, such as the spear techniques of Hozoin Ryu, the sword
techniques of Kashima Shin Ryu, and Kendo. He also studied Kodokan Judo and
Sumo Wrestling.
All of the arts listed above contributed to Aikido in some way. The principal that makes Aikido different to any other empty hand art is the fact that Aikido is a Muto Ryu Kenjutsu, a sword art without using a sword. Another principle that makes Aikido different from Jujutsu, is Aikido does not use the principle of "push when pulled, and pull when pushed," instead the principle is "ENTER when pulled, and TURN when pushed."1
References
1. Aikido. Kisshomaru Ueshiba. Hozansha Publications. 1985.
2. Abundant Peace. The Biography of Morihei Ueshiba Founder of Aikido. John Stevens. Shambhala Publications. 1987.
3. Budo. Teachings of the Founder of Aikido. Morihei Ueshiba. Kisshomaru Ueshiba. Kodansha International. 1991.
4. Invincible Warrior. A Pictorial Biography of Morihei Ueshiba, the Founder of Aikido. John Stevens. Shambhala Publications. 1997.