Aikido is a Japanese martial art developed by Ueshiba Morihei (In Japanese, the family name goes first). Ueshiba is often referred to by the honorific O-Sensei which means "Great Teacher." He developed Aikido from his studies of numerous koryu (older martial arts), but relied mainly on Daito ryu Jujutsu as taught to him by Takeda Sokaku. Sometime during O-Sensei's lifetime Daito ryu started to be called Daito ryu Aikijujutsu or Daito ryu Aikijutsu. At the same time Takeda Sensei witnessed what Ueshiba was now teaching in Tokyo, and basically said, "That's great stuff, but it isn't Daito ryu. Call it something else." So Ueshiba started calling his art, Aikibudo. It was still being changed and modified as time went on. Officially, 1942 is the start of when the art was called Aikido, though O-Sensei made reference to aikido before then.
The word Aikido is made up of three kanji. Starting at the end, many martial arts end in "Do". This means "the way of, or the path of." The first kanji, "Ai" means harmony, or joining. This is used in conjunction with the second kanji "Ki." Ki, litterally means breath, energy, or spirit. So, all together, you have ai-ki-do: "The way of Harmonizing energy," which doesn't begin to explain what Aikido is, or can be, but it's a starting point for us to break off to talk about in other blog posts.