There are some concepts that just seem inherently difficult to define in a clear way, yet most humans have an "intuition" about what they mean. These include:

* DefinitionOfLife
* Complexity (ComplexityMetrics)
* "Pattern"
* Syntax - Perhaps because it is tied to "pattern" (WhyTypeSyntax)

''Syntax, at least in ComputerScience, is '''not''' difficult to define clearly.  It is no more and no less than the rules governing the formation of sentences in a language.  This was made entirely clear on WhyTypeSyntax.'' -- DaveVoorhis