Definitions of margin:

• noun:   the blank space that surrounds the text on a page
• noun:   the amount of collateral a customer deposits with a broker when borrowing from the broker to buy securities
• noun:   the boundary line or the area immediately inside the boundary
• noun:   a strip near the boundary of an object

  Example: "He jotted a note on the margin of the page"

• noun:   a permissible difference; allowing some freedom to move within limits
• noun:   (finance) the net sales minus the cost of goods and services sold