means A thin coating or film that builds up on a surface — most commonly the sticky layer of bacteria on teeth, or the fatty deposits inside arteries; also a flat ornamental or commemorative tablet mounted on a wall.

from From French plaque, meaning a flat plate or slab, which came into French from Dutch plak, 'a flat board or tablet,' tied to the verb plakken, 'to stick or plaster.' So at its root the word is about something flat that adheres to a surface — fitting for both the brass tablet bolted to a wall and the clinging film on your teeth. The medical and dental senses are later borrowings of the same flat-and-stuck idea.