A lung is an internal organ that makes it possible for you to breathe. Your lungs remove carbon dioxide from your body while sending oxygen into your blood stream.
All vertebrates — animals with a spinal column — have lungs. Humans have two of them, and they're what makes your chest rise and fall as you breathe in and out. They're also the organs that keep you breathing, and therefore alive. Lung, by way of the Old English lungen, comes from an Indo-European root that literally means "the light organ."