The term mysticism (from ancient Greek μυστικός mystikós 'mysterious', related to myein 'to close mouth or eyes') refers to reports and statements about the experience of a divine or absolute reality as well as efforts towards such an experience. 

-- Wikipedia -- translated from German

Mysticism is popularly known as becoming one with God or the Absolute, but may refer to any kind of ecstasy or altered state of consciousness which is given a religious or spiritual meaning.

It may also refer to the attainment of insight in ultimate or hidden truths, and to human transformation supported by various practices and experiences. 

-- Wikipedia –