A pillow is something you rest your head on while you sleep. Or, during a slumber party, a pillow could be used as a weapon.
While in some cultures people have used hard pillows made of stone or wood, it's most common today to sleep with your head on a pillow stuffed with feathers or foam. Many people also have pillows on their chairs and sofas, either as decoration or to make them more comfortable. During a "pillow fight," playful combatants hit each other with soft pillows.