noun
Standard word for a rainbow; used in everyday and literary contexts.
雨が上がった後、空に大きな虹が出た。
After the rain stopped, a big rainbow appeared in the sky.
虹は七色に見える。
A rainbow looks like it has seven colors.
The kanji 虹 is a phono-semantic compound: semantic 虫 (insect, here representing a mythical creature) and phonetic 工. The reading にじ is a native Japanese word of uncertain origin.