Warmer (noun)

A device used to keep something warm, such as a heating pad or a hand warmer.

Origin:

Mid 17th century: from warm, on the pattern of pair.

Examples:

1. She put the hand warmer in her pocket to keep her hands warm.
2. The baby's bottle was placed in a bottle warmer to heat it up.
3. The camping gear included a warmer for the sleeping bag.
4. The foot warmer was a lifesaver on cold winter nights.
5. He put the food in the warming tray to keep it warm.