(Last updated, January 29, 2014)
Mysticism is the belief (-ism) that an individual can acquire "knowledge" through some means other than reason (the logical formation of abstractions from sense-perception). Mystic knowledge passes from some mystical source through a mystical way (pathway, channel, route) to the mind of the mystic. The mystic then, on faith, accepts knowledge passing to him.
SOURCES. Example sources of mystical knowledge are: God, Holy Scripture, the heart, the gut, an inner voice, authority, and the "spirit" of a place (such as a burial ground or ancient forest). "Authority" is ambiguous. It names either (a) the mystical idea of an automatically and unquestionably qualified source, or (b) the objective idea of someone who has demonstrated his ability to speak knowledgeably on a particular subject.
WAYS. Example ways of acquiring mystical knowledge are: revelation, tradition, instinct, intuition, "just knowing," "listening to the heart" (or gut), hearing an "inner voice," feeling, dogma, and "common sense" (in one of its meanings). Some of these terms are ambiguous. For various meanings of "common sense," both mystical and objective, see the April 23, 2012 post here.
NATURE OF FAITH. The term "faith" can be ambiguous. Sometimes "faith" is a broad synonym for mysticism. At other times, the term "faith" names an idea of a process -- acceptance without objective evidence -- that in fact characterizes every type of mysticism. At still other times, "faith" names a set of ideas acquired mystically ("the Christian faith"). Generally, I will be using the term faith to mean the process of acceptance of any idea—regardless of the source or the way of acquiring it—without objective proof or even in contradiction to objective proof.
When a religious mystic speaks of his faith, the mystic usually identifies the source, as with "faith in God." Some mystics dispense with the term "faith" and ignore the step of acceptance. Nevertheless they are acting on faith. In summary, then, faith is the assent to "knowledge" gained through some mystical way.
COMMON CHARACTERISTICS. What characteristics do the various sorts of mysticism share? In most cases, there are two: an initial experience and a subsequent transformation into words.
According to reports from mystics with whom I have talked, mystics experience something in a non-sense-perceptible form. Some mystics use sensory words -- such as "hear" and "see" -- to describe their non-sensory experiences, but those terms are usually metaphors.
Example conversation: Mr. A: "How did you know whether to marry Jane or not?" Mr. B: " I knew I should marry her. I felt it in my gut, and I knew that what my gut was telling me was right."
That "experience" in the mystic's mind somehow changes -- usually in some unspecified manner -- to conceptual (verbal) "knowledge". With their "knowledge" now specified in words, mystics can write and speak to others about the mystical experiences. A mystical experience untranslated into words is incommunicable.
Revelations, inner voices, and possibly tradition are apparent exceptions: The initial experience is the words. An example is a god speaking commandments directly to someone as a revelation.
DISTINGUISHING CHARACTERISTICS. What characteristics distinguish one sort of mysticism from another? I have already identified two distinguishing characteristics: (1) the sources of that knowledge; and (2) the ways (channels, paths, routes) of acquiring the "special" knowledge. Based on general reading and general experience in talking with some mystics, I suggest a third characteristic that distinguishes one sort of mysticism from another: (3) the form of the message which a particular version of mysticism delivers: Is it verbal (as in the voice of a god), visual (as in an apparition), or felt (as in feeling the presence of Satan).
SUMMARY. Mysticism is the belief (-ism) that one can acquire knowledge through some means other than reason, that is, some means other than logical abstraction from sense-perception or introspection. One sort of mysticism is distinguished from another by its alleged source (God, the Dictator, my gut, etc.), by the way in which the message is received (a revelation from heaven, an inner voice, etc.), and by the form of the message.
Author, The Power and the Glory: The Key Ideas and Crusading Lives of Eight Debaters of Reason vs. Faith
 Most of these types of mysticism come from Ayn Rand's list: Ayn Rand, "Faith and Force: Destroyers of the Modern World," Philosophy: Who Needs It, pp. 75-76 (hb). She delivered this lecture at Yale University, Columbia University, and Brooklyn College in 1960.