The term 'PART OF A.D.' is composed of two crossword solutions, ANNO and DOMINI. ANNO is a Latin word meaning 'year' and is typically abbreviated as A.D., while DOMINI is a Latin word meaning 'of the Lord' and is also abbreviated as A.D. Together, they form the phrase 'anno domini, ' which is Latin for 'in the year of the Lord' and is commonly used to refer to the Christian era. This phrase is often used to denote years in the Gregorian calendar, which is the most widely used calendar system in the world today.