lowland
Land that is low and fairly flat, often near sea level.
Lowland describes land that sits at a low elevation, usually near sea level or in valleys between higher terrain. If you've ever looked at mountains from far away and noticed the flatter areas spreading out below them, you were seeing lowlands. Rivers often flow through lowlands because water naturally moves to the lowest point it can find.
Lowlands tend to have richer soil than highlands because centuries of rainfall wash nutrients down from higher ground, making them excellent for farming. The Netherlands is famous for its lowlands (some even below sea level!), where farmers grow tulips and other crops on incredibly flat, fertile ground. In Scotland, the Scottish Lowlands are the flatter southern and eastern regions, in contrast to the rugged Scottish Highlands to the north.
The word can be used as an adjective too: lowland areas, lowland forests, or lowland gorillas (which live in lower-altitude forests rather than in the mountains). When you hear about lowlands, picture wide, relatively flat stretches of land where settlements, farms, and cities naturally develop, since building and traveling are easier there than in steep, mountainous terrain.