Ratio of gross profit to sales revenue.
(Also sometimes used as a synonym for gross profit).
For a manufacturer, gross margin is a measure of a company's efficiency in turning raw materials into income; for a retailer it measures their markup over wholesale.
Most companies would like a gross margin that's as large as possible.
An exception is the discount retailer; part of their game is to make their operations and financing so efficient that they can afford to keep their markup tiny.
See the sample income statement.