Definitions for "Irreducible Complexity"
Behe's name for the idea of a system which would stop working if any of its components was removed. Such systems evolve quite often.
The concept that systems contain components which by themselves hold no value until they are brought together into a working system. Michael Behe is a major proponent of this thinking. The common example he uses is the mouse trap which has many parts which have no individual value until they are put together as a mouse trap. The argument by analogy is that living systems contain irreducibly complex parts and therefore could not have evolved gradually.
biological complexity for which imagination fails to perceive an evolutionary origin