Thai: ?????? (ma-groot)
Ma-groot is usually called ‘Kaffir Lime’, but recently I read that ‘Kaffir’ is actually an ethnic slur(!) in South Africa and elsewhere. So instead, I’ll just use the Thai name. Ma-groot are a type of citrus, related to lime, but with a completely different smell and taste. The juice (what little there is) is not used. The skin of the fruit is used in curry pastes, and the leaves are used to flavor soups, curries and more.
It may be next to impossible to find fresh ma-groot outside of Thailand. You may be able to find frozen or dried. If frozen, double the amount called for in the recipe, as freezing the fruit makes it lose flavor. If dried, pre-soak to soften, and use three times what the recipe calls for.