There are so many varieties of mousse it is impossible to give a rule for all of them. In general, we could define a mousse as any soft or creamy dessert made light and fluffy by the addition of whipped cream, beaten egg whites, or both. Note that Bavarians and chiffons fit this description. In fact, they are often served as mousses, but with the gelatin reduced or left out so the mousse is softer.
There are many kinds of bases for mousses. They may be nothing more than melted chocolate or puréed fresh fruit, or they may be more complex, like the bases for chiffons.