What is Act Prohibiting The Return Of Slaves?

Legal Definition
The Act Prohibiting the Return of Slaves was a law passed by the United States Congress during the American Civil War forbidding the military to return escaped slaves to their owners.

As Union armies entered Southern territory during the early years of the War, emboldened slaves began fleeing behind Union lines to secure their freedom. Some commanders put the slaves to work digging entrenchments, building fortifications, and performing other camp work. Such slaves came to be called "contraband," a term emphasizing their status as captured enemy property. Other Army commanders — particularly Democrats — returned the slaves to their owners. Congress reacted by approving on March 13, 1862 an act prohibiting the military from sending escaped slaves back into slavery.
-- Wikipedia