What character is used to separate the sheet name from the cell address within a formula containing a sheet reference?

The character used to separate the sheet name from the cell addres within a formula containing a sheet reference is!

For example:

Formula without sheet reference: = A1.

Formula with reference to another sheet in the same book: = Sheet1! A1

Formula with reference to a sheet in other book: = = '[Name_of_the_sheet]'! A1

That character is inserted automately so you do not have to write it.