What does Reiki mean?

The characters that form the symbol that we know as Reiki come from the Japanese language which uses pictograms.  
Pictogram also known as pictograph or simply as picto is an ideogram that conveys its meaning through its pictorial resemblance to a physical object. Pictography is a form of writing that uses pictorial drawings. We are familiar with ancient pictograms like those showcased from ancient times  or modern pictograms like those we see on the road as pictures of an airplane by the airport, bathroom signs and so on. 

According to Wikipedia Reiki usually means ‘mysterious atmosphere; miraculous sign’ however have in mind that Wikipedia content changes and evolves like everything else, everything!

To the best of past Reiki Masters research the Japanese characters that compounds the symbol of Reiki have been translated as two stems forming the word ‘Reiki’ 

In modern times Japanese Reiki is 'written', as the picto in the picture above, in Kanji and it compounds the word that we know as Reiki.  Rei meaning spirit, miraculous, divine and Ki meaning vital energy, breath of life, consciousness. Ki is apparently the same Qi as in Qi Gong or the Chi energy that your hear from your acupuncturist Qi and Chi are frequently translated as natural energy, life force or energy flow however the literal translation appears to be breath, air or gas.→

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Most Reiki Masters & Reiki Master Teachers explain the meaning of Reiki as Universal Life Force Energy, Reiki Master Teacher Hawayo Takata who conveyed Reiki Teachings to the Western Culture used to explain Reiki as God’s Power and once again who can explain God. 
Fortunately Reiki practitioners do not need to understand Reiki completely in order to use it and deliver it.  However; it feels nice to have some knowledge. 

Personally my favorite part of Reiki is that even though is not well supported with scientific research, Reiki practitioners feel Reiki flowing through them when giving or receiving Reiki. Reiki Masters claim that the student do not find Reiki instead Reiki finds the student .