A plaque is a sticky film of bacteria that constantly forms on your teeth. You can't see it easily, but if you run your tongue over your teeth after eating, that slightly fuzzy feeling is plaque building up. When you brush and floss your teeth, you're removing this bacterial coating before it can harden and damage your enamel.

If plaque isn't cleaned off regularly, it hardens into a rough, yellowish substance called tartar that only a dentist can remove. The bacteria in plaque feed on sugars from food and produce acids that eat away at tooth enamel, causing cavities. This is why dentists recommend brushing twice a day: you're literally scraping away colonies of bacteria before they can harm your teeth.

The word also refers to a flat commemorative tablet, usually made of metal or stone, that marks an important person, place, or event. You might see a plaque on a historic building explaining who lived there, or a plaque on a park bench honoring someone's memory. Schools often display plaques listing the names of students who won academic awards.

In medicine, plaque can also mean fatty deposits that build up inside blood vessels, which is why doctors sometimes warn about “plaque in the arteries.”