Golden Ratio is a term (with an astounding number of aliases) used to describe aesthetically pleasing proportioning within a piece.

Designer’s guide to the Golden Ratio / Fibonacci Sequence. This stuff is important for hierarchy and proportion in your design

The Golden Ratio is used to mathematically describe the most pleasing composition possible in art.

What Is the Golden Ratio?: The Golden Ratio in action.

"Another simple tool for web design linked to the golden ratio is Fibonacci numbers. A Fibonacci sequence begins with 0 and The previous two numbers are added together to produce the next number in the sequence: and so on.

Use mathematics to create flawless designs.