Reiki has existed on this planet since ancient time. Only limited people with intensive training or certain groups of people through secret rituals being passed on from ancestors had known about it and could access it to use for healing. Reiki was re-discovered by a Japanese Buddhist named Mikao Usui in 1922.
He was a successful businessman, researcher, Buddhist monk and later became a well known healer. He was in pursuit of the way to heal others without depleting his own energy for many years. He was also searching for the meaning of life. He had an enlightening experience on the 21st. day of fasting on Mt. Kurama, Kyoto Japan, when an intensive energy entered the top of his head and knocked him down unconscious for hours. It was March of 1922 and he realized he had received a healing power he had wished for.
He discovered a way to be able to pass on the ability to heal by a method called attunement. The Reiki system Mikao Usui discovered, theorized and taught is referred to as "Usui Reiki" or just "Reiki" some times. He established Usui Reiki Ryoho Gakkai (Healing Society) in April of 1922.
One of the masters and very likely the last master Mikao Usui initiated was Chujiro Hayashi. He trained and initiated Hawayo Takata from Hawaii to be a master. Takata took it back to Hawaii and spread it. Since she deviated the teaching from the traditional way Mikao Usui passed on through the Usui Reiki Ryoho Gakkai (Healing Society), it is called Western style Reiki, Western Usui Reiki or Western Reiki.
After the death of Hawayo Takata in 1980, Western Reiki was taken to Japan and spread out there while few in the world knew that the traditional Usui Reiki Ryoho Gakkai still existed although as a small and somewhat hidden association.