When rendered, a whitespace character does not correspond to a visual mark, but typically does occupy an area on a page. As is common in technical literature, the two words "white space" have found widespread usage as the single term "whitespace", especially when used as an adjective, as in "whitespace character". (Wikipedia)

Preprocessor directives must appear as the first non-whitespace character on a line (eggheadcafe.com)

You can only filter nodes that contain only whitespace. In a text node that contains any non-whitespace characters, whitespace is considered significant, and is always displayed. (IBM Corporation)