"A.D." stands for "Anno Domini," a Latin phrase meaning "in the year of our Lord." It is used to label years in the Gregorian calendar, which is the calendar system most widely used today. The system counts years from the traditional date of the birth of Jesus Christ, with years before this event labeled as B.C. (Before Christ). The term "A.D." was introduced by the monk Dionysius Exiguus in the 6th century. It helps to organize historical events chronologically, allowing for a clearer understanding of timelines in history, religion, and culture. The use of A.D. is common in historical texts, religious documents, and academic studies.