The answer is Workbook.
This question is a bit confusing but the keyword here is the ENTIRE excel file.
A spreadsheet is another name for a worksheet. A worksheet is the primary document in a single page of a file that you use in Excel to store and work with data. It contains those cells organized by rows and columns. A workbook, on the other hand, is the name given to an excel file that contains one or several worksheets.
Thus, strictly speaking, when you launch an Excel program, what loads is an empty workbook that consists of one or more blank worksheets available for use.