Closure

Closure: when objects are not completely defined, the eye fills them in and you perceive them as whole objects (frequently without realizing it).

This panda is not completely defined, but your eye fills in the lines and "closes" the body of the panda:

And here the "M" in the logo is defined by an absence of outlines, but you can clearly see the letter:

Designing a graphic using this principle can add visual interest to your pages.