Portrait mode and landscape mode refer to the orientation of text (and pictures) on a printed page. (The paper must be a rectangle, however in practice square sheets are hardly ever used.) The term was most often found in print setup dialog boxes of personal computer systems, until it became common to replace them with little icons showing the same idea.
In portrait mode the text is printed on the paper such that the reader will turn the long side of the paper vertical and the short side horizontal. Like most portrait photos. In landscape mode on the other hand, the long side is horizontal, and the short side vertical. Like most landscape paintings.
The terms can also be used to indicate the orientation of a computer display. Most displays are landscape nowadays, but portrait displays were popular for a time around 1990, as they could display the image of a whole (portrait mode) sheet of paper.
