the ones I know, in the code:

""
Empty
Null

but within Excel (in formulas), the only way I know is "".

you can also create a name and say it means "". And then use the name intead
of the two quotes.

to do it, you type Ctrl+F3
name: Blank (the word blank)
refersto: =""

it should work!

i imagine that maybe you want to be able to replace "" in bunch of formulas
to something, and it would be easier to use a name instead.

:)