A period of 10 years. Since there was no year 0, the years 1-10 correspond to the first decade, 11-20 to the second, etc. A new decade therefore technically begins in the years 1991, 2001, 2011, etc. However, rather than wasting time arguing about whether 2000 or 2001 is the start of the "new" decade, calling 2000 the start of the next decade seems reasonable to me, simply because that's when the calendar changes the decades digit!