kaffir, kieffer lime, makrut lime leaves, or magrood
Native to Southeast Asia, wild limes are small, bright green citrus fruits with bumpy, wrinkled skin. Both the peel and the pulp deliver vibrant citrus flavor and are deeply aromatic, but even the leaves are good for cooking. Used widely in Southeast Asian cuisine, especially in curries, wild lime leaves give dishes a refreshing, lingering lift that is intensely floral and citrusy.
You can try substituting lime zest. It doesn't have that same intensely floral aroma, but it will give the dish a similar refreshing citrusy flavor.
Fresh lime leaves are often sold in Asian markets, or if they're not available fresh, frozen or dried leaves are sometimes sold. Whenever you can, unless otherwise directed, opt for the superior flavor and aroma of fresh lime leaves (frozen are your next-best bet).
Fresh wild lime leaves should be refrigreated in a sealed plastic bag until ready to use. They'll keep like this for several days. Keep frozen lime leaves frozen until just before using.