A mitten is a cold-weather piece of clothing that you wear on your hand. Unlike gloves, which cover each finger individually, mittens cover your four fingers together, and your thumb separately.
Mittens keep your hands warmer than gloves, because your fingers share their warmth and expose less surface area to the cold. Small children often wear mittens — they're easier to put on than gloves are, and it's common for them to be attached to each other with yarn that can be threaded through the arms of a coat, or clipped onto a coat's sleeves. The word mitten comes from medietana, "divided in the middle" in Vulgar Latin.