The term 'calendar number' refers to a numerical representation of a DATE. It is often used in computer programming, where dates are represented as a number of days since a certain reference point, such as the beginning of the year or the start of a particular era. Calendar numbers are also used in some calendar systems, such as the Chinese calendar, where each day is assigned a unique number. The crossword solution for the term 'calendar number' is 'DATE', which is a four-letter word that describes the concept of a calendar number.