Expansion means making something bigger, wider, or more extensive. When you blow up a balloon, you're watching expansion in action: the rubber stretches and the space inside grows. When a company opens new stores in different cities, that's business expansion.

The word appears everywhere in science, history, and everyday life. Water undergoes expansion when it freezes into ice, which helps ice cubes float (frozen water is less dense and takes up more space than the same amount of liquid water). The universe itself is in a state of expansion, with galaxies moving away from each other like dots on an inflating balloon.

Throughout history, territorial expansion has shaped nations. The westward expansion of the United States in the 1800s brought settlers across the continent to the Pacific Ocean. Ancient Rome built an empire through military expansion across Europe, North Africa, and the Middle East.

You might experience expansion in your own life when your school adds a new wing, when your vocabulary expands through reading, or when a crack in the sidewalk gets wider (concrete expansion and contraction from temperature changes cause this). The opposite of expansion is contraction, when something shrinks or gets smaller. Understanding expansion helps explain everything from why bridges have gaps in them (allowing for expansion on hot days) to how your lungs work (they expand when you breathe in).