What is the definition of "warm front"?

A "warm front" is the leading edge of an advancing warm air mass that is replacing a retreating and relatively colder air mass. A warm front is located within a broader trough of low pressure than a cold front.

A warm front moves more slowly than a cold front that usually follows them; this is because cold air is denser and therefore more difficult to remove from the Earth's surface. The warm air mass behind a warm front is usually higher in humidity than the colder air preceding it. As the warm air mass rises into regions with lower pressure, the air expands and cools. The phrase warm front is often used in weather reports.