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Here's a very objective and powerful way to measure complexity: dependencies and volatility.

Otherwise we're all saying "complex" but not being clear and likely meaning different things.

For example, a lot of people believe that "not easy" = "complex" but as Rich Hickey articulates that's a counterproductive way to think of complexity. (See http://www.infoq.com/presentations/Simple-Made-Easy)

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"dependencies and volatility" But what does this even mean? I'm okay with using Rich Hickey's definitions. But I don't recall that in Rich's talk.

If your system's design results in your stable components depend on the non-stable (volatile) components, your system is complex. This is because volatile components change often and these changes ripple to your stable components effectively rendering them volatile. Now the whole system becomes volatile, and the changes to it become very hard to reason about - hence complex.

Avoiding this problem has been captured, among others, by the Stable Dependencies Principle (http://c2.com/cgi/wiki?StableDependenciesPrinciple), which states that the dependencies should be in the direction of the stability. A related one is the Stable Abstractions Principle (http://c2.com/cgi/wiki?StableAbstractionsPrinciple), which states that components should be as abstract as they are stable.

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