In the past I had the same question when attempting various Asian dishes which called for lime leaves. Through research and experience I found that these recipes are referring to the ****** lime (aka makrut lime) as others have surrmised.
These are the fragrant leaves of the wild lime tree and are used widely in Thai and South East Asian cuisine in the same way as bay leaves are used in the West. The double leaves are joined tip to end, creating an unusual figure-of-eight shape. They have a spicy, lemony flavor and give a distinctive citrus scent to soups and curries. They are becoming more widely available, both fresh and dried, but substitute lemon grass if you can’t find them.
While the fruit and zest of the ****** have plenty of flavor, the leaves are the most prized for their floral scent that straddles the line between lemon and lime. The leaves are generally inedible and, like lemongrass stalks or cinnamon sticks, are fished out of soups and curries before serving. However, finely sliced fresh leaves can be used for garnish.
The ****** lime leaf is essential to Thai curries and soups, but many dishes can benefit from their flavor. Fry in a bit of oil for a fragrant ****** oil to be used to finish salads or soups. ****** lime leaves can also be steeped in milk that can then be used in fragrant puddings and custards.
Ground leaves are an easy way to add a floral citrus flavor to soups, curries, stews, and to homemade curry pastes. Blending it with ground chilies, ground lemongrass, salt, black pepper, and amchoor powder will create a fantastic spice blend for chicken or pork that is spicy, citric, sweet, savory, and sour.
If you make your own sausage, add a bit of ground ****** lime leaf to the next batch. You’ll be blown away by their Thai-flavored intensity when they hit the searing metal of the grill.