Software which requires the user to obtain and abide by the terms of a license from the copyright holder. Most commercial software is proprietary.
An issue which users should be aware of – particularly with a proprietary Content Management System (CMS) is data portability or the ability to move your data (and website) from one platform to another. With a proprietary system you may be “locked into” a particular vendor and find it difficult and expensive to move your existing site to a new hosting solution.
See also: Open Source