From AMS Glossary
A flash of red light seen on or (seemingly) adjacent to the lower rim of the low sun (at either sunrise or sunset).
Much of what is said about the green flash applies to the red flash, only transferred to the base of the sun and applied to multiple images of the lower red rim. During a sunset, an island of red light will sometimes form just below the rest of the sun, proceed to join the main disc, and then progress up the sides of the sun as serrations. The red flash is not perceived as being as striking an event as the green flash if for no other reason than that its color is more characteristic of the low sun.