Any Lawful Permanent Resident who has been outside the United States and is returning to the U.S. Also defined as a "special immigrant." If outside of the U.S. for more than 180 days, must apply for readmission to the U.S. If outside of the U.S. for more than one year and is returning to his or her permanent residence in the United States, usually must have a re-entry documentation from USCIS or an immigrant visa from the Department of State.
Source: U.S. Citizenship and Immigration Services