"In his well-known lectures on "The Varieties of Religious Experience", William James listed the two main characteristics that defined for him the mystical state: ineffability and the noetic quality. Ineffability stems from the fact that the experience defies expression. It cannot be transmitted to anyone who has not had a similar experience, and communication with one who has is made only through vague expressions rather than explicit description. The noetic quality is the insight revealed through the experience, an insight that is also hard to describe. However, the truths understood by a person in a mystical state, although inexpressible, may have an extraordinary effect on his or her entire life."
James mentions two additional characteristics: transiency and passivity. Transiency is the normal brevity of the experiences, which rarely lasts for more than a few hours and often passes in moments. Nevertheless, an exquisite moment is often experienced as eternal.
The characteristic of passivity is the inability to enter the mystical experience at will, or remain in it through our own efforts. Wanting is not enough to assure its presence - but we will see that a high level of desire and effort makes us more receptive to the experiences when they come our way.
Source: "Silence, Simplicity and Solitude - a Guide for Spiritual Retreat" by David A. Cooper