As described in What is Reiki?, Reiki is life energy. It is universal or higher wisdom energy.

In Japanese health philosophy, you might have healthy energy genki, or you might have unhealthy energy byoki. Genki is normal and natural and flowing. Byoki is blocked and stagnant. You might think of it like water: Flowing water is healthy while stagnant water is not. We can see the latter in that stagnant water starts allows for mosquito breeding and the growth of unhealthy plant...