Reiki is the transmission of universal life force energy through a practitioner to the client. 

The word "reiki" comes from two Japanese words:

Rei translates as "universal"

Ki (or chi) translates as "life force energy"

Life force energy knows where it is needed. An attuned Reiki practitioner transmits energy through the "laying on of hands." There does not have to be any contact between the client and practitioner. The client (generally) lays face-up. The practitioner allows energy to flow through the chakras in the their hands to the client. Life Force energy is then transmitted to the client. It is important that the client be completely grounded before they leave at the end of a session. It is also important that the client drink plenty of water as Reiki can be very detoxifying at first. Water will help to promote the process and keep the client well hydrated.