White / Negative Space (Design Dictionary)

White space is better described as the negative space of a page layout, illustration or sculpture. It is somewhere which has been left unmarked like "margins, gutters, and space between columns, lines of type, graphics, figures, or objects drawn or depicted." (Wikipedia).

White space is useful for allowing the elements of your design space to breathe so the overall effect is generally more professional looking.

There is a lovely post on white space on Eva Black's blog which you should check out and the post she recommends - Design Terms : White Space and Space Yourself – Smashing Magazine.

Related reading:
Why is White Space Good For Graphic Design - Designmodo
White Space in Graphic Design, and Why It's Important | Printwand™
11 Reasons Why White Spaces are Good in Graphic Design | Naldz Graphics