leap year

Short definition from Wiktionary

Noun

1. 1
   A year in the Julian or Gregorian calendar with an intercalary day added to February (in the Gregorian calendar, February 29), used to adjust for the extra hours of the solar year; a 366-day year.
2. 2
   Any other year featuring intercalation, such as a year in a lunisolar calendar with 13 months instead of 12, used to maintain its alignment with the seasons of the solar year.