Dander refers to tiny flakes of dead skin that shed from animals with fur or feathers. These microscopic particles float through the air in homes with pets, settling on furniture, carpets, and clothing. When people say they're allergic to cats or dogs, they're usually reacting to proteins in the animal's dander, not to the fur itself.

The flakes are so small you can't see individual pieces, but if you've ever noticed dust floating in a sunbeam in a room where pets live, some of that dust is dander. Birds produce it too: parrots and other pet birds release dander as part of keeping their feathers healthy.

You might also hear the phrase “get your dander up,” which means to become angry or irritated. If your teacher gets her dander up about students talking during silent reading, she's becoming genuinely annoyed. The phrase stuck around even though most people forgot the original meaning, so now dander primarily refers to those skin flakes that can make some people sneeze.