After practicing Reiki for 20 years and teaching Reiki for almost 16 years, I have an even deeper appreciation for how inadequate words are in capturing the essence of a Reiki treatment. 
Reiki is a holistic healing art that is best experienced rather than explained. 

When I first learned Reiki, I was taught, like many others, that Reiki is a “universal energy” which is “channeled” through the practitioner's hands to the recipient. It was explained that the person receiving the Reiki treatment activated the energy when the practitioner’s hands were gently placed along areas of the body (or a few inches above.) 

With practice, the Reiki practitioner soon realized that he or she could not “push or put" Reiki in - for lack of better terms.  Giving a Reiki treatment is more a process of letting go and trusting that a healing environment is being established.  My own experience is that Reiki cannot be manipulated - it can only be humbly facilitated.  Perhaps a treatment is most aptly described as fostering an environment of unconditional acceptance that creates a gentle, non-intrusive, safe space where the recipient can tap into their deepest wisdom and healing capacity. This healing space supports the well-being of the client in all areas of being – body, mind and spirit.  Because this healing space extends to the practitioner, you often hear practitioners say they receive a treatment while giving one. Self-treatments create this same nurturing environment.

I invite you to ponder how you would describe Reiki if you have either received a Reiki treatment, or are are a Reiki practitioner yourself. And do semantics even make a difference? They might in terms of Reiki being discussed with the general public or in a professional health care setting. 

Some find it best to describe Reiki as a spiritual healing practice because it often creates a meditative state. Meditative states support our ability to better know ourselves and our relationship with the world around us which in turn is deeply healing. (The term spiritual varies from religious beliefs and has a broader trans-personal context in this definition.)

Bottom line. I deeply respect everyone’s right to view and explain Reiki in a way that is helpful for them. Perhaps all of us who love and practice Reiki are simply trying to point to this beautiful healing art with different eyes. Much like the fable of the three blind mice describing an elephant, maybe Reiki is larger than we realize and we are each pointing to a different aspect of what is true for us individually.  I am just grateful for the gift of Reiki practice and the opportunity to explore its profound depth.

With all these thoughts in mind, you might find it interesting to explore these two interesting links below.  
http://reikirays.com/1344/co-rekatuib-of=reiki-and-science/

2010 small study designed by well-known Reiki Master/author, Pamela Miles,  along with Yale University School of Medicine researchers on the effects of Reiki on heart rate variability:  http://reikiinmedicine.org/pdf/jacc.pdf