There are a few cases where words are ambiguous. We use particular words to try and avoid this.
This is the document or page which you are imposing from . It is not normally modified, but instead used to create or overlay the target.
Page and page number has its usual meaning, but when performing imposition it usually refers to pages in the source.
A sheet is just a page in the target document. The word sheet is used rather than page, so it is clearer which document we are talking about.