Chad, a 'master set' is one which includes all commonly-known, systematic variation groups. (Condition of the cards is irrelevant.)
Lately a favorite we've been discussing on this board has been R114 US Caramel Presidents, a good example of this. R114 came in red, orange and blue background versions, with portraits of the presidents of the USA. While a normal set would typically have one card for each president, regardless of background color, a master set would have a card for each president, in each color (ie, three times as large as a normal set).