Churn rate is also sometimes called attrition rate. It is one of two primary factors that determine the steady-state level of customers a business will support.

In its broadest sense, churn rate is a measure of the number of individuals or items moving into or out of a collection over a specific period of time.

The term is used in many contexts, but is most widely applied in business with respect to a contractual customer base. For instance, it is an important factor for any business with a subscriber-based service model, including mobile telephone networks and pay TV operators. The term is also used to refer to participant turnover in peer-to-peer networks.

The phrase is based on the English idiom "to churn up", meaning to agitate or produce violent motion.
http://en.wikipedia.org/wiki/Churn_rate

Churn Rate for a SaaS Provider:

So a healthy SaaS business will have a combination of a small churn and relatively high upsell rates, so the total is more than 100% renewal value (e.g. 88% renewal rate + 15% upsell).
http://gadishamia.wordpress.com/2008/09/27/how-to-measure-a-good-saas-company/

The beauty of a SaaS model is that it provides companies with a very stable and predictable revenue stream. Public companies and public company investors love this.

That being said, I think you need to be more specific about what industry and/or application you are talking about. If you say, what is the churn rate for SaaS based businesses, I'm not sure anyone can give you a great answer. It really get at the heart of what you are looking, it would help if you are more specific about what you want. If I told you, the churn rate for SaaS businesses is 22% annually, it is like an onion (a rough weighted average with many layers that probably isn't very useful to you). Can you be more specific and maybe I can be a bigger help.