the difference that exists between net sales and the cost of merchandise sold and that is taken as that from which expenses must be met or profit derived or from which other obligations must be met or other advantages derived called also gross margin (MW)
see the Definition from http://www.investorwords.com/cgi-bin/getword.cgi?2944
too complicated, there are various margin. profit margin is one type. but margin is for sure not equal to margin ratio. Make sense?