the reformed Julian calendar now in use, according to which the ordinary year consists of 365 days, and a leap year of 366 days occurs in every year whose number is exactly divisible by 4 except centenary years whose numbers are not exactly divisible by 400, as 1700, 1800, and 1900.

British Dictionary definitions for gregorian calendar

Gregorian calendar

noun

the revision of the Julian calendar introduced in 1582 by Pope Gregory XIII and still in force, whereby the ordinary year is made to consist of 365 days and a leap year occurs in every year whose number is divisible by four, except those centenary years, such as 1900, whose numbers are not divisible by 400