Custom Dimensions have been available using one name or another in paid web analytics products for years. In Omniture (Adobe) SiteCatalyst (Analytics) their called eVars. Webtrends has offer custom dimensions since the early/mid-2000s.

Google Analytics offered five “custom variables” at first, then this Universal Analytics notion came along and now we can have 20 custom dimensions now. (I’m grateful!)

These variables can be used to track visitor interactions in a truly customized manner tailored to your website. They allow you to count what you want, not what everyone else using standard Google Analytics is counting. Combine custom dimensions with custom events and Google Analytics will be custom for your site forever more! Take a look at this blog post at analytics-ninja and learn more. See also this reference entry on Google’s Developer site which explains Custom Dimensions and Custom Metrics in a way developers will appreciate.