WYSIWYG is a pronounceable acronym for "What you see is what you get". It is used to describe an editor that allows you to see what a document looks like while you are editing it.
 Word processing feature
When word processing editors first appeared on computers they used tagged files where tags were added surrounding text to define specialized handling for text such as when text size needed to be changed or it needed to be displayed in bold or italics. Paragraph styling, such as indenting, justification, etc., would also be defined using codes. To see what the document would actually look like you needed to preview the page on the screen. However, if you then want to change something you would need to switch back to editing mode.
With WYSIWYG editing the paragraph styling and word styling always looks exactly as it would look on the page when it was printed. As a matter of fact the idea was whatever you see on the screen will look exactly the same as it would appear when it was printed. The codes used to make the display are hidden on the screen display but are still in the document.
 Web page processing
As web pages came into being a tagged format called HTML was developed to provide the editing control of the presentation. The term WYSIWYG was still used but generally a page might look a little different from the edited page due to the user having control over the font size and page width. The length scrolled as needed to see the full document.
Some people wanted more control and wrote their pages to mimic exactly the source and required the user to scroll both up and down as well as sideways to accomplish a page display.
 eBook processing
eBook formats are typically based on the same tagged format used for HTML but the display of an eBook generally does not use scrolling but has fixed size pages that probably does not match the page size of the source document. While the term WSYIWYG is still used to describe an eBook editor, the writer may still have to preview the pages on actual reading programs or devices to ensure the results are OK. Most eBook formats work this way including some favorites such as AZW, MOBI and ePub. These formats are called reflowable since the page size is fixed by the device screen size not by the program. The paragraph content may spread across more lines with shorter lines if the device screen is smaller than the source was expecting just as it would if you were to move the book contents to smaller sized paper.
Users may also be able to override some of the source settings such as justification, font sizes, and margins. For these reasons the original source, even when edited with a WYSIWYG editor may look considerably different in practice.
Some eBook formats use a fixed page size format to force the actual page to be as completely controlled as it is in a printed book. Formats such as PDF, CBR CBZ, or DjVu. There are now also Fixed layout ePubs. EBook readers often use a shrink to fit approach on these documents with varying success. For these books the term WYSIWYG when applied to an editor would have the traditional meaning. Reading a fixed size book with an eBook reader smaller than the intended screen size would require either smaller text by shrinking the text and pictures or you would have to move the page around on the screen to read for edge to edge.