Embolismic

an embolismic year or month

To predict Easter computists needed to develop a cyclical table based around a common multiple m of solar years and lunar months: that is, one in which a whole number of solar years equated to a whole number of lunar months. The general idea was that m years after some reference year, Easter will be on the same date as in the reference year itself because a whole number of lunar months will have passed. The number of lunar months in those m solar years wouldn't be an exact multiple of 12 (since there are more lunar months than solar ones), so the period wouldn't equate to a whole number of lunar years. For this reason, an extra month, called an embolismic month, was added to some years in the lunar calendar to make sure that years counted in lunar months would not gradually creep more and more ahead of years counted in solar months.