What is the common dipping sauce for tempura called?

The common dipping sauce for tempura is called tentsuyu.

Tentsuyu is a light Japanese dipping sauce traditionally made from dashi, mirin, and shōyu, the Japanese name for soy sauce. A frequently cited proportion is roughly three parts dashi to one part mirin and one part soy sauce, though restaurants and home cooks adjust the balance to taste.

Tempura is usually served hot and immediately after frying, while its batter is still crisp. Diners dip each piece briefly into tentsuyu, often accompanied by grated daikon, which adds freshness and a mild sharpness. The sauce is not the same as plain soy sauce: dashi and mirin give it a deeper, sweeter, more rounded flavor.

Ponzu can also accompany fried foods, and salt is another common choice, but neither is the standard tempura dipping sauce. Tentsuyu may also be used with tempura served over rice or noodles, although those dishes often use a thicker, sweeter sauce.