Mousse means foam (or moss). It can be sweet or savoury, soft and light, or the spit-like stuff that you get instead of sauce at some very high end restaurants.
Savoury or sweet mousses are usually chilled in a mould. Classic savoury ones (shellfish, poultry, vegetable) are often served as a hors d’oeuvres or an entrée.
Sweet ones can be any flavour—coffee, vanilla, lemon, strawberry and the famous mousse au chocolat. You can also find mousse glacée or mousse givrée (frozen or frosty mousse).
Attend a bal mousse (foam party) at your own risk (and don’t wear anything that shouldn’t get wet).
Images by Edsel Little, Fetebeaupont