The term perpetual beta refers to the way that web systems are often released without the final stages of polishing and testing that would have been typical for a shrink-wrapped piece of software. The term was coined around 2000 as one of the characteristics of Web2.0. Now-a-days most downloaded apps have automatic updates, and so aspects of perpetual beta often apply to them also.
Perpetual beta is a quality of the product or service being delivered, but it is often associated with agile development processes.
Used in Chap. 6: page 81