Tonjiru (豚汁), literally meaning “pork (ton) soup (jiru)”, is basically miso soup featuring pork and root vegetables. The name came from pork/pig (豚) and soup (汁) because the key ingredient of this soup is pork. The kanji character “豚” can be read as “ton” or “buta” as the Japanese reading of kanji usually has two (sometimes more) ways, i.e. the Chinese reading and Japanese reading. As such, some people call it Butajiru (豚汁) too, just another way to read the same kanji character.
This is quite different from the usual miso soup as miso soups are normally just vegetables with no oil in them. Tonjiru does not get cold as fast as other miso soups because the surface of the soup is covered by a very thin film of oil from the pork. This film of oil keeps the soup hot, which is perfect on a cold day.
Because of the fat content in this soup, it is a cold-weather staple, but I do enjoy the soup all year-round.