A letter of the alphabet, in the range A to G, attached to a year. It signifies the date of the first Sunday in that year (A=January 1….G=January 7) and hence the alignment between day of the week and date for the year.
There are only seven distinct years in this system; leap years are given a double dominical letter (for example, 2000 is BA). This signifies that the part of the year before 29 February is a B year, but in the part following that day and date are aligned as though it were an A year.