Air Mass
An air mass is a large body of air with roughly uniform temperature and humidity throughout, named for the surface region where it formed.
Definition
An air mass is a huge body of air, often bigger than a state, with roughly uniform temperature and humidity throughout, picked up from the land or ocean surface it sat over for days to weeks under stagnant high-pressure conditions. Air masses are classified by the Bergeron scheme using their moisture source, maritime or continental, and their latitude of origin, arctic, polar, tropical, or equatorial, with the four types most relevant to the United States being continental polar (cold, dry), maritime tropical (warm, humid), continental tropical (hot, dry), and maritime polar (cool, moist); where two different air masses meet, a front forms. Air masses also modify as they travel, warming and moistening or cooling and drying depending on the surface they cross, which controls the weather generated within them.
Example
A cold, dry air mass sliding down from Canada in winter can drop North Carolina temperatures below freezing for days, while Charlotte's summer weather is dominated by warm, humid maritime tropical air from the Gulf of Mexico and Atlantic. A continental polar air mass leaving Canada at $-30^\circ C$ warms and moistens crossing the Midwest but still arrives in the Carolinas cold and dry enough that its sharp contrast with warm Gulf Stream air offshore makes the coast a favored zone for rapid nor'easter development, and cold air wedged against the Appalachians produces the Piedmont's notorious freezing rain.
Key Insight
Air masses behave like visitors carrying the weather of their home region with them, Gulf air bringing warmth and humidity, Canadian air bringing cold and dry conditions, and continental polar air crossing the Great Lakes even picks up enough moisture to dump lake-effect snow on the far shore. Because a single air mass can sit over the eastern United States for several days, weather tends to arrive in multi-day episodes rather than changing at random, shifting only when the next air mass moves in.