I guess you mean Beethoven's Middle period, as this is the only context I've heard this term applied to. The Waldstein sonata IS in his middle period, but it doesn't start it. His 3rd symphony is the work that kicks off the second period. Expanded sonata forms, increased orchestral dynamics and new orchestration, and Beethoven's big, grand, and emotion charged middle period. I think the tempest may have been the start of his middle period, but don't hold me to that, as I'm really not sure. Hope this helps!