“Theme creep” is what I call it when functionality that has nothing to do with the presentational layer of a WordPress website “creeps” into the theme. What this ends up doing is chaining you to a WordPress theme that seemed like a beautiful and great one for your site when you first saw it. Chains you so that in six months or thirty, when you find yourself wanting a visual change of pace you’re left with a terrible choice: your pretty new look or your properly functioning WordPress site.
It’s a very accurate term for crawling over the boundaries of presentation and into the realm better served by plugins. This always makes me feel uneasy about WordPress themes perfectly suited for the task at hand: what if the development stalls, there will be no support for new WordPress features? What will it take going to reproduce the functionality, presuming that the theme permits to edit the code?