How is the smallest erasable unit of flash memory called?
On the NAND flash chips I've seen, a page is the amount of information that is typically written at a time, and a block is the smallest amount that can be erased.  Some chips have sectors which consist of multiple blocks, and provide commands to erase an entire sector at once.  I've never found such commands very useful, though, since they seem to take almost as long as erasing blocks individually.

I would suggest using terms "programmable page" and "erasable block" to refer to those two different sizes of entity.  The term "sector" would not generally be used in vendor-independent discussion, since different vendors define sectors differently.  While programmable pages are usually of one uniform size, and erasable blocks are usually of another, many manufacturers use a mixture of large and small sectors, even within the same device.