Tom Barrett has a new post that shares an opinion he has about WordPress and what it can be used for - that the popular software is more than just a blog, it's a PHP framework.

A software library is a collection of resources used to develop software. [...] A Software Framework is a software library with certain goals in mind [...]. Hopefully theme and plugin developers will be familiar enough with WordPress to see where I am going with this.

He mentions some of the similarities WordPress has with other PHP frameworks including the integration of hooks/filters/plugins, working with core files that are maintained by an external entity and that it's a full-functional CMS out of the box.

From this, I conclude that WordPress is a framework. It provides us with a library of resources that allow us to extend and specialize it into the site and CMS that we want.