Glossy double-lobed leaves from a knobbly Southeast Asian lime, holding a concentrated floral lime oil that is quite different from lime juice.
What it does in a recipe: A high, floral citrus perfume that survives simmering — it flavours coconut curries and soups from the start rather than the finish. Pick the swap that covers that job — the ratios below are written so you can act on them without doing any arithmetic.
The central rib is tough. Fold the leaf in half, tear the rib out, and shred what remains as finely as you can if it is going into the dish rather than being fished back out.
Cooking around an allergy? Diet tags here describe the ingredient itself, not any particular brand. Processed products change formulation without notice and shared production lines are common, so read the label on everything you use. A severe allergy is a medical matter — confirm with the manufacturer or your clinician rather than relying on a substitution chart.