What is 1991 Armed Forces Immigration Adjustment Act?

Legal Definition
The Armed Forces Immigration Adjustment Act 1991, also known as the Six and Six Program, was enacted on October 1, 1991. The Act amended the Immigration and Nationality Act of 1965, also known as the Hart-Celler Act.

The Armed Forces Immigration Adjustment Act allows aliens who have served in the United States Armed Forces for at least period of 12 years to be granted special immigrant status. Immigrants who have served for 6 years may also obtain special immigrant status if they re-enlist so their total service commitment is 12 years. Special immigration status may also be awarded to those who have a "recommendation by the executive department under which such alien served or is serving."

There are no numerical limits on how many special immigrant status visas can be issued. Special immigrant status visa holders are able to obtain permanent resident status, and the same applies to their spouses and children.

Some applicants for special immigrant status are also immediately eligible for naturalization as US citizens.
-- Wikipedia