In Genesis, a rainbow is the sign of God's covenant with Earth after Noah's Flood.
Genesis 9 describes God making a covenant with Noah, his descendants, and every living creature that another flood would not destroy all life. The rainbow is placed in the clouds as a visible reminder of that promise. The covenant therefore extends beyond Noah's family to the wider living world.
The passage is an important biblical explanation of rainbow symbolism, but it does not claim that rainbows first physically appeared at that moment. Modern science explains a rainbow as the result of light being refracted, reflected, and dispersed by water droplets. The religious narrative gives the phenomenon a covenant meaning.
The rainbow covenant follows Noah's departure from the ark and the offering he makes. It should not be confused with the dove's olive leaf, which appears earlier as a sign that the waters are receding.