(1) The division of something conceptually into two opposed or contrasted aspects, or the state of being so divided. (Stanford Encyclopedia of Philosophy 2011)
(2) In the history of thought, the idea that, for some particular domain, there are two fundamental kinds or categories of things or principles. (Stanford Encyclopedia of Philosophy 2011)
Stanford Encyclopedia of Philosophy. 2011. s.v. "Dualism." Accessed on September 11, 2012. Available at http://plato.stanford.edu/entries/dualism/.
The yin yang concept in Chinese philosophy emphasizes the interaction between dual elements and their harmonization, ensuring a constant dynamic balance often through a cyclic dominance of one element and then the other, such as day and night (see Principles of Systems Thinking)