The word Reiki is represented by two kanji, Rei and Ki. Rei is Spiritual Wisdom. The general definition is universal or present everywhere. Ki is Life Energy. It means the same thing as Qi in Chinese or Prana in Sanskrit, but science calls it biofield energy. It is the non-physical energy that animates all living things.
When your life energy is low, or if there is restriction in its flow, you will be more vulnerable to illness. But when it is high and flowing freely, you are less likely to get sick. Ki is also the primary energy of our emotions, thoughts, and spiritual life.
Ki or Qi is the non-physical energy used by all healers. It is present all around us and can be guided by the mind. It is guided by spiritual consciousness. All healers can use Ki or life energy, but not all healers use Reiki. Reiki can only be channeled by someone who has been attuned to it.
Reiki is not taught in the way other healing techniques are taught. It is transfered to the student from a Reiki Master during the attunement process. During the attunement, the Rei or spiritual consciousness makes adjustments in the studen't chakras and energy pathways, and links the student to the Reiki source.
Because Reiki is guided by spiritual consciousness, it can never do harm. Ki can become stuck in the body for a variety of reasons, on of them being negative thoughts or feelings. The Reiki energy works with the negative thoughts and feelings that have created a block on one's flow of healthy Ki and replaces them with healthy thoughts and feelings. Reiki raises the vibratory level in and around the physical body where the negative thoughts and feelings are attached. This causes the negative energy to break and fall away.
When a block is released sometimes a person will feel a cold sensation. Or when it is melted or burned up a hot sensation. Once healthy Ki is flowing naturally the physical irgans and tissues are able to complete their healing process.
Reiki is both powerful and gentle. It can treat pretty much everything and is always beneficial. It also can work to improve the effectiveness of all other types of therapy. A session feels like a wonderful, glowing radiance.
Resource: Reiki: The Healing Touch By: William Lee Rand