PageWidth is an option for output streams and for cells which specifies how wide each line of text should be allowed to be.
Possible settings for both output streams and cells are: SetOptions[
] resets the line width allowed for an open stream.
Possible settings for cells only are: PageWidth->WindowWidth allows each line to use the full width of the displayed window, taking into account settings for CellMargins.
See the Mathematica book: Section 2.10.10, Section 2.11.3.
See also: TotalWidth, TextJustification, AutoIndent.
This opens a stream, specifying that the page width is 20 characters.
This writes out an expression to the stream.
This closes the stream.
The lines in the expression written out are all broken so as to be at most 20 characters.
x^5 + 5*x^4*y +
5*x*y^4 + y^5