1. coalsack - Noun
2. Coalsack - Proper noun
A sack for carrying coal.
(astronomy) Any of the spaces in the Milky Way which are very black, owing to the nearly complete absence of stars.
Coalsack
The dark nebula near the constellation of the Southern Cross.
Explorer Amerigo Vespucci seems to have observed not only the Southern Cross but also the neighboring Coalsack Nebula on his second voyage in 1501–02. Source: Internet
Deep-sky objects The Coalsack Nebula is the most prominent dark nebula in the skies, easily visible to the naked eye as a prominent dark patch in the southern Milky Way. Source: Internet