A bissextile month is another term for a leap month. It is a month that is inserted into a calendar to ensure that the calendar year stays synchronized with the astronomical year. The term bissextile is derived from the Latin word "bis sextus," which means "twice sixth." Some synonyms for bissextile month include intercalary month, embolismic month, and leap month. In some cultures, a bissextile month is referred to as a "13th month" or a "leap year month." Different calendars have different rules for calculating the need for a bissextile month. For example, the Chinese calendar adds a leap month in some years to keep its lunar year aligned with the solar year.