It is the portion of a page left unmarked: margins, gutters, and space between columns, lines of type, graphics, figures, or objects drawn or depicted, and is not necessarily actually white if the background is of a different colour.
The term arises from graphic design practice, where printing processes generally use white paper.
A page full of text or graphics with very little white space may appear cluttered, and be difficult to read.
[1] Some designs compensate for restricted white space by use of leading and typeface.
Extensive white space may be left intentionally, with the purpose of giving an appearance deemed classic, elegant, or rich.