Plain English Meaning
How much of the radar's signal bounces back from precipitation. Bigger or denser drops bounce back more signal, which shows up as brighter, hotter colors on the radar picture.
Full Definition
In weather radar, the measure of how strongly a target — such as raindrops, snowflakes, or hail — returns the radar's transmitted energy back to the receiving antenna. Higher reflectivity values indicate larger or more numerous precipitation particles, and are displayed on radar imagery as more intense colors (typically yellows, oranges, and reds).
Context Anchor
Seen on weather radar displays and radar observation figures, where different colors or shading show areas of weaker or stronger weather returns.
Derivation
From Latin reflectere, meaning 'to bend back.' In radar, it describes how much energy is bent back toward the antenna after striking a target — useful because the term is built around the idea of returning signal, which is exactly what the radar measures.