Mousse is also (and I think this preceeds the hair product) a light and fluffy food concoction that can be either a desert or a fruit/vegetable or meat/fish dish.

They are made fluffy by adding whipped cream, whipped egg whites and/or whipped gelatin to a smooth base ingredient.

Commonly seen as a desert is chocolate mousse. The most common fish mousse I know of is salmon mousse.