An attempt made by Julius Caesar to adapt the calendar year to the time actually required for the earth to make one complete revolution around the sun.
He estimated that time as 365.25 days, and therefore every fourth year he added a day to the 365 days of the year.
His calendar was inaccurate because the time for the earth's journey is a little shorter than 365.25 days.
This was superseded in 1582 by the Gregorian Calendar devised under the direction of Pope Gregory XIII.
New Catholic Dictionary