To invade means to enter a place forcefully and in an unwelcome way, usually to take control or cause harm. When one country invades another, it sends military forces across the border to conquer territory or overthrow the government. History is full of invasions: the Norman invasion of England in 1066, Napoleon's invasion of Russia in 1812, and the Allied invasion of Normandy in 1944 that helped end World War II.

The word applies to smaller scales too. When ants invade a kitchen, they swarm in searching for food. When your little brother invades your room without permission, he's barging into your private space. Your privacy can be invaded when someone reads your diary or constantly interrupts you.

An invasion is the act of invading, and invaders are those who invade. The word carries a sense of unwelcome intrusion: nobody invites an invasion. Even in sports, when a team plays on their opponent's home field, announcers might say the visitors are “invading” the stadium.

Notice the difference between invading and simply entering or visiting. You visit a friend's house when invited, but you invade their space when you barge in uninvited and make yourself at home.