This was the designation of Japanese mercantile convoys (together with a numerical suffix) between Singapore and Imari Bay of Moji in Kyushu in the Japanese home islands (10 July 1943/February 1945).
Merchant ships from Moji, Kaibokan and Sasebo formed met in Imara Bay to be organised into southbound convoys carrying supplies and equipment for the Japanese forces fighting in the Burma campaign. On their northbound return passages, the convoys carried food, petrol products and raw materials to Japan from the captured European colonies of the Dutch East Indies, French Indo-China, and British Malaya and Burma. Japan was slow to get off the mark with the establishment of any sort of convoy system, and these 'Mi' convoys were begun only in the middle of 1943 in an attempt to protect fast, high-value tankers and troopships from the improved offensive capabilities of US submarines using the Mk 14 torpedo, and of the warplanes of increasingly aggressive US carrier task forces.
The convoys were routed through the East China Sea, Strait of Formosa Strait and South China Sea. Ships often joined or left the convoys at the Formosan ports of Takao and Keelung, at the Mako naval base in the Pescadores islands group, and at the Indo-Chinese ports of Cap Saint Jacques and Cam Ranh Bay. Some convoys stopped at Manila until the introduction of 'Mata' and 'Tama' feeder convoys between Manila and Takao enabled the 'Hi' convoys to avoid US wolfpacks in the Strait of Luzon by hugging the Asian east coast between Hainan and Shanghai.
The first of the convoys was the southward 'Hi-1' of 10/19 July 1943, and the last was the northward 'Hi-98' which departed Singapore on 27 February 1945 with only the Ryoei Maru, which was sunk by the US submarine Bashaw.
The northward 'Hi-88' of 20 January/18 March 1945 in fact comprised an agglomeration of 10 smaller convoys.