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 output streams are:
SetOptions[stream, PageWidth -> val] resets the line width allowed for an open stream.
Possible settings for cells 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 and Section 2.11.3.
See also: TotalWidth, TextJustification, AutoIndent.