Defined narrowly, a visionary is one who experiences a vision or apparition connected to the supernatural. At times this involves seeing into the future. Certain people are capable of going into the visionary state via meditation, drugs, lucid dreams, day dreams, or art. One such example is Hildegard of Bingen, a 12th century artist/visionary.
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 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) 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 Jobs. 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.
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.