Century = 100 miles
Metric Century = 100Km or 62.137Miles
Generally, the milage needs to covered within a 1 day period, to be considered a "true" century.
I've done 5 metric centuries in the past year. If it doesn't rain this Sunday, I might try for another one.
By the end of the year I plan to complete a 100mile ride, to see if it is possible.