What is the correct term for the smallest erasable unit of flash memory? If there is no official one, which word could be used in vendor-neutral text?
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.
There is no 'correct' term - different manufacturers have different conventions. The most common one I've seen is sector for erase and page for program, but this is by no means universal.
To add to the confusion, some flash memory (cf. ST Micro's STM32L) is
protected by sector erased by page written by word (or half-page)
... and sometimes the pages or blocks on some devices varies (ST Micro's STM32F4xx series) with memory address. This also occurs on NAND flash for doing things like have a bootloader.