"If man is to survive, he will have learned to take a delight in the essential differences between men and between cultures. He will learn that differences in ideas and attitudes are a delight, part of life's exciting variety, not something to fear." - Gene Roddenberry

IDIC (from the Star Trek Concordance, 1976):

The most revered Vulcan symbol, combining variations in texture, shape, and color.  The name is Terran, an acronym for "Infinite Diversity in Infinite Combinations"; the Vulcan name is not known. (This has changed since 1976, see below)

Spock's IDIC, which can be worn as a pin or as a pendant, is a circle and triangle of white and yellow-gold metals in shiny and textured (or Florentine) finish, with a white jewel. Spock explains that the different shapes and materials represent the diverse things which come together to create truth and beauty; the glory of creation lies in its infinite diversity and meanings. ("Is There in Truth No Beauty?")

Kirk recalls to the Spock-clone the philosophy of the Vulcan IDIC and what it means.  He also asks if an army of Spocks could impose peace on the galaxy and make other beings accept the Phylosian philisophy in defiance of the Vulcan IDIC concept.  The Spock-clone decides that it cannot be done. ("The Infinite Vulcan")

The above text is taken from The Star Trek Concordance by Bjo Trimble, 1976

From the vulcan-l Vulcan FAQ:

Steve Boozer has this additional item:

According to the unnamed Vulcan Master who tutored teenaged Tuvok, it is called kol-ut-shan - "the cornerstone of our beliefs" [VOY "Gravity"].
In fandom the IDIC is also called t'triahve.
In Traditional Golic Vulcan the term used is Va'Vuhnaya s'Va'Terishlar. Another term for the concept is Kol-uchang.