Definition 3, "a type of cake" seems wrong to me. I don't think I've ever heard a sponge cake called "a sponge", and if I answered the question "What's for dessert?" with "A sponge.", I'd expect people to understand I meant definition 2, despite the context clues.
In my experience, people call that dessert "a sponge cake", never "a sponge".
- Maybe it's a British thing. See, for example, 
- Why not past? Rklawton 04:56, 21 August 2006 (UTC)