Using different widths for different characters. In a proportionally spaced font, the letter I is narrower than the letter q and the letter m wider. This book uses a proportionally spaced font, as do most books, magazines, and newspapers.
The opposite of proportional spacing is monospacing. In a monospaced font, each character has the same width. Non-graphics display screens, display text in a monospaced font. Almost all printers, with the exception of line printers, are able to print with either proportionally spaced or monospaced fonts.