Lime leaves (sometimes called “kaffir lime leaves”) are used in Southeast Asian cooking, particularly in Thai curries, stir fries and soups. They are tough, leathery leaves that are usually added whole and removed before eating. They can also be ground to a fine powder in a spice grinder, a technique that is especially useful for adding lime leaves to spice blends.
In terms of flavour, lime leaves have a very distinctive citrusy taste, not necessarily limey, and not quite lemony. If you do any Thai cooking, there’s really no replacement, and even if you’re using a commercial green curry paste, adding a lime leaf or two to the pan can really boost the authenticity of the flavour.