What is the Japanese broth made from kelp called?

The Japanese broth made from kelp is called kombu dashi.

Kombu dashi is a clear, plant-based stock prepared by steeping dried kombu, an edible kelp, in water. The kombu may be soaked in cold water for several hours or gently heated until the water is nearly simmering. It should generally be removed before a hard boil, because prolonged boiling can make the broth bitter or unpleasantly slimy.

Kombu supplies glutamic acid, one of the compounds responsible for umami. In the better-known awase dashi, kombu is combined with katsuobushi, or dried skipjack-tuna shavings, whose inosinate creates a complementary savory effect. Kombu dashi itself contains no fish, making it useful in vegetarian and vegan Japanese cooking, although diners should still check packaged products for added ingredients.

The answer is not simply “dashi,” because dashi is a broad family of Japanese stocks. Ichiban dashi usually means a first extraction, while katsuobushi dashi identifies a stock centered on dried fish. Kombu dashi specifically names the kelp-based variation. Kombu is also used in dishes such as tsukudani, and its umami helped Japanese chemist Kikunae Ikeda identify the taste later named umami in 1908.