black flag

Short definition from Wiktionary

Noun

1. 1
   Any flag that has a black, or predominantly black, color, (especially): A flag with a black color, used as a symbol of anarchism.
2. 2
   Any flag that has a black, or predominantly black, color, (especially): A racing flag with a black color, used in auto racing to summon a driver to the pits.

Verb

1. 1
   To summon (a racing driver) to the pits by waving a black flag.