Sushi rice. すしめし。
All types of sushi are made with vinegared sushi rice - this forms in fact the criterion whether something can be called "sushi" or not.
In Japan, apprentice chefs spend a few years only learning how to make good sushi rice. Here are the rules of thumb. The rice should be firm, so not soaked after rinsing, and cooked with somewhat less water than normally - if the rice is too wet, it will not absorb the vinegar dressing after cooking. To add flavor, when cooking sushi rice chefs often add a strip of kelp (konbu) and a splash of sake. The dressing is made with rice vinegar (su), sugar and salt. The ratio of these three ingredients is a well-guarded secret of each sushi chef, and there are also regional differences: in Kyoto quite a lot of sugar is used, while some chefs in Tokyo almost use no sugar at all.
When cooking at home, a good ratio to start with (which later can be adjusted according to taste) would be: 6 table spoons of rice vinegar to 2 table spoons of sugar to 2.5 teaspoons of salt - this for an amount of rice of about 4 rice cooker cups. Dissolving the sugar and salt takes some time - if in a hurry do this faster over low heat, but afterwards cool down the mixture to room temperature. After cooking the rice, it is put into a wooden hangiri tub (or a wooden or even glass salad bowl, but never metal) and tossed with a rice paddle or wooden spoon. At the same time, sprinkle the dressing over the rice. Use it liberally, but not so much that the rice gets musty. Cool the rice with a hand fan while you keep tossing it. It should be at room temperature when you start making sushi. Sushi rice can never be kept longer than a day - to keep it for a few hours, cover the tub with a wet cloth. Never refrigerate.