The book "Pro Spring" has a pretty good definition on p 3:
"Spring is described as a lightweight framework...the lightweight part of the description doesn't really refer to the number of classes or the size of the distribution, but rather, it defines the Spring
philosophy as a whole -- that is, minimal impact(emphasized). Spring is lightweight in the sense that you have to make few, if any, changes to your application code to gain the benefits of the Spring core, and should you choose to discontinue using Spring at any point, you will find doing so quite simple."
Ben