A calendar is a leap month calendar if:
- it divides time into years which are more-or-less in sync with seasonal (a.k.a. tropical) years,
- it divides years into months (which may or may not correspond with lunations),
- most years have a fixed number of months but
- in some years there is an additional month which is added in order that calendar years stay in sync with seasonal years.
The extra month (which is not always the 13th month) is called either a leap month or an intercalary month (due to intercalation within the other months).
By analogy with the concept of leap year in the Gregorian calendar a year which has a leap month may be termed a leap year, although the term long year (in contrast to a normal year) is better in order to avoid confusion with calendars which use a leap day rather than a leap month.
When a calendar has months which are intended to stay in sync with lunations the number of months in normal years is 12, with occasionally a 13-month year.
Since (as shown by the example of the Gregorian calendar) a month does not have to correspond to a lunation, the number of months in a normal year can be other than twelve, e.g., ten 36-day 'months' and fourteen 26-day 'months'.
There are many leap month calendars, which insert the intercalary month in a variety of ways. Leap month calendars include (with numbers of months shown):
- Chinese Lunar Calendar 12 or 13 (approx)
- Meyer-Palmen Solilunar Calendar 12 or 13 (approx)
- Hermetic Lunar Week Calendar 12 or 13 (approx)
- Kalacakra Calendar 12 or 13 (approx)
- Pontisso Simple Lunisolar Calendar 12 or 13 (approx)
- Thirteen Month Leap-Month Calendar 13 or 14
- 26-day Month Calendar 14 or 15 (approx)
- 36-day Month Calendar 10 or 11 (approx)
- Dove Calendar 21 or 22(approx)
- Leap month variants of the Bahá'í Calendar 19 or 20 months of 19 days
- Rectified Hebrew Calendar 12 or 13 months of 29 or 30 days