The civil year, according to the correction introduced by Pope Gregory XIII. in 1582. The equinox which occurred on the 25th of March, in the time of Julius Caesar, fell on the 11th of March in the year 1582. This was because the Julian calculation of 365 1/4 days to a year was 11 min. 10 sec. too much. Gregory suppressed ten days, so as to make the equinox fall on the 21st of March, as it did at the Council of Nice, and, by some simple arrangements, prevented the recurrence in future of a similar error.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Gregorian Year from Fact Monster: