What is the Key Difference Between Nougat and Meringue?
The basic difference is consistency: meringue tends to be lighter, airier, and drier, while nougat is more chewy.

This is achieved by different ratios of sugar to egg whites. Nougat has significantly more sugar in proportion to egg whites. (Compare this nougat recipe, ~4 cups sugar/honey to four egg whites, and this meringue recipe, 1/4 cup sugar to four egg whites.)