1. A bus that has two levels, one positioned above the other. Can be used as a noun or an adjective. We took a double-decker bus tour of the city while we were there on vacation.

2. A sandwich that has two layers, one on top of the other. Can be used as a noun or an adjective. Wow Mom, you made me quite a double-decker for lunch! I don't think I can eat it all.