Wainewright's latest post is likely to find the meaning amid the SaaS confusion. An exerpt:
The truth is that SaaS is, and always will be, a very broadly defined term, and therefore it's inevitable that there will be many different subcategories of SaaS. That creates plenty of potential for misunderstanding and confusion, of course. But don't worry, there's an easy way to avoid getting bogged down. The key to making sense of SaaS is see it in terms of the journey a software developer or architect might take as they plunge further and deeper into the SaaS model. At the outset, the product they'll create looks very similar to conventional licensed software. At the end of their journey, it may not look like software at all.From there, Wainewright goes through the various flavors--hosting, shared instance, shared services, communities of interest and pay as you go--currently under the SaaS umbrella and concludes that we just may need a new acronym as things evolve. Definitely worth a read.