An agreement during the Constitutional Convention of 1787 protecting the interests of slave owners by forbidding Congress the ability to tax the export of goods from any State, and, for 20 years, the power to continue on with the slave trade.

The Slave Trade Compromise stated that Congress couldn't ban the slave trade until 1808. It was done so that the Constitution would be accepted by the slave states. If the Constitutional Convention had banned the slave trade, the southern states would probably not have consented to the Constitution.

 

Representatives and direct Taxes shall be
apportioned among the several States which may be included within this
Union, according to their respective Numbers, which shall be determined
by adding the whole Number of free Persons, including those bound to
Service for a Term of Years, and excluding Indians not taxed, three fifths