Tartar is a hard, yellowish substance that forms on teeth when plaque isn't brushed away. Plaque is the sticky film that builds up on your teeth throughout the day, and if you don't brush it off regularly, it hardens into tartar (also called calculus). Once tartar forms, your toothbrush can't remove it anymore. Only a dentist or dental hygienist can scrape it away with special tools during a teeth cleaning.

Tartar matters because it can lead to cavities and gum disease. It's rough and porous, which means bacteria love to live in it, and it often forms along the gum line, where it's hardest to clean. This is why dentists emphasize brushing twice a day and flossing: these habits remove plaque before it has a chance to harden into tartar.

There's also tartar sauce, a tangy condiment often served with fish, but that's a completely different word that comes from the French sauce tartare.