The word Reiki comes from two Japanese words:
Rei ~ spiritual wisdom and consciousness. This is the wisdom that comes from divine universal consciousness that is all knowing.
Ki ~ Life energy; vital life force. This is the nonphysical energy that animates all living things. If your life energy is low or there is a restriction in the flow of it, you are more vulnerable to illness and disease. If it is high and flowing freely, disease and illness will be less likely. Ki is the primary energy of our emotions, thoughts, and spiritual life.
Reiki was officially named by its founder, Mikao Usui, in the late 1800's. However, energy healing has been prevalent since the beginning of time. Reiki is just one of many healing energies that are granted to us by the Divine Universe. We can tap into them at any given time. Jesus was a powerful energy healer and when he ascended, he left for us the beautiful Christ Consciousness energy that we can access anytime through spiritual awareness and enlightenment.
We are all energy beings and have the power to heal ourselves. An energy practitioner is a conduit or channel for these healing energies to pass through so that they may be received by others. A Reiki master practitioner goes through three levels of "training" with a Reiki master practitioner teacher over a period of three years. Energy workers and/or healers do not necessarily need training, but it goes something along the lines of how students learn from their guru. The power to heal is already inside, however it is always beneficial to have that teacher to guide and help along the path. (Energy and light workers need energy and light workers!)
Let's look a little more specifically about what Reiki is.
In addition to what Reiki is, it is important to point out a few things that Reiki is not.