What in the Hell is a Spandrel?

Spandrel: 1.) is the space between two arches or between an arch and a rectangular enclosure.

What does that have to do with anything you might wonder? Well, for some reason I have heard that word crop up again and again in discussions on evolutionary theory. In it's technical sense, spandrel refers to the space created (unintentionally) when the actual physical structure of an arc is formed. But what it represents when used in the context of evolutionary theory is more along the lines of:

Spandrel: 2.) a trait created unintentionally, as the result of some other trait which has evolutionary value. (Note: This is not an official definition, it is just the definition as I understand it to be.)

Using the architectural example the spandrel (space between wall and arch) has no value, and doesn't do anything besides allow the arc to be the shape necessary to form it's function. The evolutionary parallel is a trait/quality that was seemingly developed "by accident" or as a byproduct of some more useful trait. For example, the brain did not evolve to a larger size to accommodate language. But as a result of a larger brain (i.e. the arc) language (the spandrel) was able to develop.