Wiktionary
n. the minimal amount of complexity a system may exhibit while still maintaining its primary function
Wikipedia
Irreducible complexity (IC) is a pseudoscientific argument that certain biological systems cannot evolve by successive small modifications to pre-existing functional systems through natural selection.*.
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Central to the creationist concept of intelligent design, IC is rejected by the scientific community, which regards intelligent design as pseudoscience. Irreducible complexity is one of two main arguments used by intelligent design proponents, the other being specified complexity.
Michael Behe, a professor of biochemistry at Lehigh University, first argued that irreducible complexity made evolution purely through natural selection of random mutations impossible. However, evolutionary biologists have demonstrated how such systems could have evolved. There are many examples documented through comparative genomics showing that complex molecular systems are formed by the addition of components as revealed by different temporal origins of their proteins.
In the 2005 Kitzmiller v. Dover Area School District trial, Behe gave testimony on the subject of irreducible complexity. The court found that "Professor Behe's claim for irreducible complexity has been refuted in peer-reviewed research papers and has been rejected by the scientific community at large."
Usage examples of "irreducible complexity".
The example of the bacterial flagellum quickly became a cornerstone of the intelligent design movement, and Behe's principle of irreducible complexity was promoted as an unavoidable barrier to the evolution of complex structures and functions.