ann-, anni-, annu-
(Latin: year, yearly)
If she marries within about nine months from the death of her husband and a child is born, a doubt might arise as to its paternity. Such a marriage is not illegal.
Great achievements or disasters, or the like; the London fire of 1666 and the plague in the same year are examples of such a year. This phrase is also used to indicate a year in which figures of great importance were born, especially when that year produced important people in great numbers.
Pronunciations for annus mirabilis are AN nuhs mee RAHB buh luhs and AN nuhs mi RAHB buh liss.
It appears that there are many who do not understand the meaning of annus mirabilis in the context of "an extraordinary year" or a "remarkable year" which covers disasters; as well as, positive happenings. If this is true, then do we need annus horribilis for greater clarification?
It was, in another context, Her Majesty Queen Elizabeth II of the United Kingdom of Great Britain and Northern Ireland who got it right: 1992 was supposed to be the European Community's annum mirabilis brought about by the Maastricht treaty and ending in the single market. Instead, it has been an annus horribilis (a dreadful, or horrible, year) with one faux pas after another, and with the cause of European unity looking like an increasingly elusive grail as the year progressed.
The plural form is anni horribiles, "dreadful years", or "horrible years".
2. Occurring every two years; biennial.
A biannual event occurs twice a year; a biennial event takes place once in two years.
To avoid confusion, it is better to use the prefix semi- to mean "twice each" (semiannual; semimonthly; semiweekly) or the phrase "twice a" or "twice each "(twice a month; twice a week; twice each year ["biannual"]), and for the other sense to use the phrase "every two" (every two months; every two weeks; every two years [or in this case, "biennial"]).