An invader is someone who enters a place by force where they're not wanted or don't belong. When an army invades another country without permission, those soldiers are invaders. Throughout history, invaders have crossed borders to conquer territory, seize resources, or expand their empires. The Roman legions were invaders across much of Europe, and the Vikings were feared as invaders along European coasts.

The word carries a sense of unwelcome intrusion. You might call wasps invaders when they build a nest in your attic, or describe an invasive plant species as an invader because it crowds out native plants. In sports, people sometimes call the visiting team the invaders, though this is usually playful rather than serious.

In video games, space invaders are classic alien enemies trying to attack Earth. When you're deeply focused on reading and someone interrupts, you might think of them as invading your concentration. The key idea is that an invader pushes into territory or space where they have no right to be, often causing disruption or harm to whoever was there first.