King James Bible Dictionary

Caused by the reflection and refraction of the rays of the sun shining on falling rain. It was appointed as a witness of the divine faithfulness (Genesis 9:12-17). It existed indeed before, but it was then constituted as a sign of the covenant. Others, however (as Delitzsch, Commentary on Pentateuch), think that it "appeared then for the first time in the vault and clouds of heaven." It is argued by those holding this opinion that the atmosphere was differently constituted before the Flood. It is referred to three other times in Scripture (Ezekiel 1:27, 28; Revelation 4:1-3; 10:1).

A token that the earth shall no more be destroyed by flood
Genesis 9:8-16; Ezekiel 1:28

Symbolic
Revelation 4:3; Revelation 10:1
Meteorology and Celestial Phenomena

the token of the covenant which God made with Noah when he came forth from the ark that the waters should no more become a flood to destroy all flesh. The right interpretation of (Genesis 9:13) seems to be that God took the rainbow, which had hitherto been but a beautiful object shining in the heavens when the sun's rays fell on falling rain, and consecrated it as the sign of his love and the witness of his promise. Ecclus. 43.11. The rainbow is a symbol of God's faithfulness and mercy. In the "rainbow around the throne," (Revelation 4:3) is seen the symbol of hope and the bright emblem of mercy and love, all the more true as a symbol because it is reflected from the storm itself.

RA'INBOW, noun A bow, or an arch of a circle, consisting of all the colors formed by the refraction and reflection of rays of light from drops of rain or vapor, appearing in the part of the hemisphere opposite to the sun. When the sun is at the horizon, the rainbow is a semicircle. The rainbow is called also iris.

The moon sometimes forms a bow or arch of light, more faint than that formed by the sun, and called lunar rainbow Similar bows at sea are called marine rainbows or sea bows.