NiMH is a kind of battery you can charge again and again. The letters mean nickel-metal hydride: nickel is a metal, and hydride means hydrogen is packed into another metal inside the cell. For years these were the usual rechargeable AA and AAA batteries in cameras, torches, toys, and small gadgets. Some older hybrid cars used big NiMH packs too. They replaced an older cousin, NiCd (nickel-cadmium), which used a nastier metal. Though actually, many new phones and cars have moved to lithium batteries, which are lighter. You still buy NiMH cells for household things you already own. People say the letters slowly, or just “nim.”