霊 (Rei-) means something mysterious, miraculous and sacred.  
気 (Ki) means the atmosphere, vapour, air, breath, vital force, energy, wind.
Reiki can be defined as a universal and sacred energy of the universe which sustains all life, also being a subtle, refined level of ki.   Ki has a variety of names all over the world from ancient times. It is called ‘Chi/Qi’ in China, ‘Ki’ in Japan and Korea, ‘प्राण-Prana’ in India, ‘Mana’ in Hawaii and Polynesia, 'རླུང-Lung’ in Tibet, ‘Orenda’ of Iroquois, ‘אור-Ohr’ in Kabbalah, etc. It is the essential energy for all beings to exist and live healthy and happy lives.

Usui Mikao Sensei (1865-1926) was bestowed with the profound insight of Reiki in 1922 at the age of 57 when he entered 安心立命 (anshin ritsumei - spiritual peace) while fasting and meditating on Mt Kurama in Kyoto; then founding the Usui Reiki Ryoho Gakkai in Tokyo to spread his knowledge and help others to heal and, with the ultimate goal of entering anshin ritsumei. Ryoho is the method, chiryo is the private treatments, koshu is for training to become a practitioner.