Google Sheets kinda flattens everything out into "sheets" doesn'it - the document and a document part can end up being called the same thing.
Excel steered "workbook" "worksheet (sheet)" here, but I remember Excel users I interviewed when doing UX work ending up at "it's all sheets" just the same.
(I remember people also calling them "tabs", and Excel UI does use that language sometimes - "sheet tabs." I don't know if Sheets does.)
