A splinter is a narrow, pointed sliver that breaks off something larger. If you walk barefoot on a wooden floor, dock, or boardwalk, you might get a splinter of wood in your foot. Ouch!
We often use the word splinter to refer to tiny shards of wood that lodge under the skin, but a sliver of any hard material — stone, glass, bone, wood, metal — can be called a splinter. When something splinters, it breaks into individual bits. You can also use the word figuratively to describe something that separates in a violent or forceful way, like when a mainstream political party splinters into more or less extreme groups.