tychism (?ta?k?z?m) n (Philosophy) philosophy the theory that chance is an objective reality at work in the universe, esp in evolutionary adaptations[from Greek tukh? chance]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014tychism1. the theory that chance is involved in evolution and that variation within a species is accidental.2. the belief that chance rather than mere determinism operates in the cosmos. Cf. uniformitarianism.See also: Evolution-Ologies & -Isms. Copyright 2008 The Gale Group, Inc. All rights reserved.