From Lexicanum:
The Great Crusade was a brief age of rebuilding and reunification following the complete regression of mankind during the Age of Strife. It was a time when the Emperor still lived in the conventional sense and led his race in person. It followed the Emperor's conquest of Terra in the Unification Wars and lasted roughly from the conquest of Earth's moon to the Battle of Isstvan III and the beginning of the Horus Heresy.