Reiki can be defined as a massage only if we adopt a very loose definition of massage. You see, in reiki the hands do not actually touch the body.
Reiki is described as "a form of alternative medicine, developed in 1922 by Japanese Buddhist Mikao Usui, [which] has been adapted across varying cultural traditions. It uses a technique commonly called palm healing or hands-on-healing. Through the use of this technique, practitioners believe that they are transferring "universal energy" through the palms of the practitioner, which they believe encourages healing*".
This energy is known as qi, a "life force energy" which is present in several Asian cultures - its existence, as you probably know, has not been proved by Western science. More than two-thousand years ago the Chinese wrote about what they called linjqi 靈氣 - the two characters are often translated into English as mysterious force, or similar expressions.
A treatment, to quote the US reiki website, feels like a wonderful glowing radiance that flows through and around you. Reiki treats the whole person including body, emotions, mind and spirit creating many beneficial effects that include relaxation and feelings of peace, security and wellbeing. Many have reported miraculous results.
You'll have to try it to make up your own mind....