Visionary

Defined broadly, a visionary, is one who can envision the future. For some groups, this can involve the supernatural or use of mind-altering drugs.
A visionary state can be achieved via meditation, drugs, lucid dreams, daydreams, or art. One example is Hildegard of Bingen, a 12th century artist/visionary and Catholic saint. Other visionaries in religion are Mohammed (who had a vision of and communed with the Angel Gabriel), St. Bernadette (who had a vision of contact with the Blessed Virgin), and Joseph Smith (who had a vision of contact with the Angel Moroni).
Extended meanings.
A "vision" can be political, religious, environmental, social, or technological in nature. By extension, a "visionary" can also be a person with a clear, distinctive and specific (in some details) vision of the future, usually connected with advances in technology or social/political arrangements. For example, Ted Nelson or U.S. Vice President Al Gore is referred to as a visionary in connection with the Internet.
Other visionaries simply imagine what does not yet exist but might some day, as some forms of "visioning" (or gazing) can provide a glimpse into the possible future. Therefore, visioning can mean seeing in a utopian way what does not yet exist on earth, but might exist in another realm, such as the ideal or perfect realm as imagined or thought. Examples are Buckminster Fuller in architecture and design, and some of the pioneers of personal computing such as Steve Wozniak. Some people use mathematics to make visionary discoveries in the nature of the universe. In that sense, a visionary may also function as a secular prophet. Some visionaries emphasize communication, and some assume a figurehead role in organizing a social group.
In art.
Artists may produce work loosely categorized as visionary art for its luminous content and/or for its use of artistic techniques that call for the use of extended powers of perception in the viewer: (e.g. Gustave Moreau, Samuel Palmer, Jean Delville, Ernst Fuchs, the French Symbolist Odilon Redon, Brion Gysin, Max Ernst, Stanley Spencer, Edward Burne Jones, Adolf Wolfli, Fred Sandback, William Blake, Hieronymus Bosch, and Henry Darger).
Visionary art can be incorrectly defined as a category of primitive art (art of those not formally trained) rather than describing people who have used their visions (or dreams) to create their paintings. Salvador Dali is one artist who would exemplify visionary art that is neither religious nor primitive.