A Cul-de-sac

1. A blind alley or dead end street.
2. A circular area at the end of a dead end street to allow cars to turn around, designed so children can play on the street, with little or no through-traffic. https://en.wikipedia.org/wiki/Dead_end_(street)
3. An impasse.
4. (medicine) A sack-like cavity, a tube open at one end only.