the·ism (th???z??m)n. Belief in the existence of a god or gods, especially belief in a personal God as creator and ruler of the world.the?ist n.the·is?tic, the·is?ti·cal adj.the·is?ti·cal·ly adv.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.theism (??i??z?m) n1. (Theology) the form of the belief in one God as the transcendent creator and ruler of the universe that does not necessarily entail further belief in divine revelation. Compare deism2. (Theology) the belief in the existence of a God or gods. Compare atheism[C17: from Greek theos god + -ism]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014the?ism (??i ?z ?m) n. 1. belief in one God as the creator and ruler of the universe, without rejection of revelation (disting. from deism). 2. belief in the existence of a god or gods (opposed to atheism). [1670?80;