WYSIWYG
What You See Is What You Get — the editor shows the document as it will look, not as marked-up code.
Before it, text was written with formatting codes interspersed and the result only appeared when printed. Bravo, written by Charles Simonyi at Xerox PARC in 1974 for the Alto, was the first word processor to show typefaces, styles and page breaks directly on screen. The phrase itself was borrowed from a well-known American television catchphrase and stuck as an industry term.
The principle now sits in everything from Word to modern web editors, but it is not free: what is shown is always an approximation, and the gap between screen and printer, or between two screen sizes, is the source of much of the work in publishing tools. The counter-movement survives in Markdown, LaTeX and other formats where the markup is the document.