Ponzu is a citrus dressing made with the grated rind and juice of a kind of a sour Japanese citrus fruit called yuzu. The yuzu is about the size of a tangerine, with a unique, aromatic lemon-lime flavor.

Fresh yuzu is only occasionally available in Japanese stores in the United States, so it's usual to substitute lemon or lime here. You can also buy bottles of ponzu sauce in Japanese markets.

Here's a recipe I found for homemade ponzu sauce.

Ponzu Sauce

1/4 cup rice vinegar (the unseasoned kind)
1/2 cup freshly squeezed lemon juice or a mixture of lemon and lime in a 2:1 ratio
1 to 2 tablespoons of finely grated fresh tangerine peel
1/3 cup tamari or soy sauce*
1 tablespoon mirin (syrupy rice wine -- optional)

*Shoyu can substitute for tamari if the Ponzu will be used quickly, as it will turn the sauce cloudy.

Combine all ingredients. Let stand overnight, then strain through a paper coffee filter before using. This makes a little over 1 cup of sauce. Store any leftover sauce refrigerated.

Ponzu sauce is used as a dipping sauce for grilled or fried fish or shellfish, meat, or tofu, or as a kind of salad dressing (mix with sesame oil if desired) for green salads or pasta.

To my knowledge, it is used throughout Japan and is not a regional product.

Susan, Hawaii