I agree with the above definition of
Reiki.
However, I would like to add that it can easily be thought of as Love. Love is healing. Love is positive, and cannot harm. A practitioner can give Reiki to anybody, at any time, but if the receiver doesn't want the Reiki, they won't get the Reiki.
Reiki: "Universal Life Energy"
The energy present in every living thing.
Loving, healing energy.
Reiki is spirit, not a religion. Reiki doesn't go against any religions, in fact it may strengthen a person's religious belief. When thinking of the Higher Source, you may think of any name you want, such as God, or Allah, or Nature... Whatever you want to call this higher flow that we live in.