1.NounA day that is an exact number of years (to the day) since a given significant event occurred. Often preceded by an ordinal number indicating the number of years.
"Today is the fiftieth anniversary of the end of the war.; We are celebrating our tenth anniversary today."
2.NounToday is the fiftieth anniversary of the end of the war.
3.NounSuch a day that commemorates a wedding.
"We are celebrating our tenth anniversary today."
4.NounA day subsequent in time to a given event by some significant period other than a year (especially as prefixed by the amount of time in question).