Perilla leaves
Large, serrated leaves of Korean perilla, a plant of the mint family, strongly aromatic with a note somewhere between mint, anise and fresh grass.
Perilla leaves, kkaennip in Korean, come from a variety of perilla, a member of the mint family; they are palm-sized, rounded with a serrated edge and slightly fuzzy. The flavour is distinct and unmistakable, minty and anise-like with a slightly bitter, grassy edge, and it holds its own against garlic and chilli. Because the leaves are firm yet pliable, they work well for wrapping meat and rice, for marinating in seasoned sauces or for frying in batter; finely shredded, they season a dish like a herb. You will find them fresh in Asian grocery stores, often labelled sesame leaves even though they have nothing to do with sesame, and also pickled in cans or jars. Perilla is also easy to grow from seed in a garden or on a balcony.
Substitute:
There is no equivalent substitute. Japanese shiso, a related variety, comes closest, but it is smaller, more delicate and tastes brighter and more lemony. For wrapping you can fall back on lettuce leaves, but then the very aroma that defines the dish is missing.