Tripel (also Trippel), or triple ale, was originally a naming convention used by Belgian Trappist breweries to describe the strongest beer in their range, however, the name is now used by a number of brewers around the world to describe a strong ale.
Tripel indicates a relative strength in a range of beers. So, the trappist beers were divided into three: enkel, dubbel and tripel (basic, double and triple). Considering the importance of the Holy Trinity in the church, it is unlikely that the choice of three types of beers was accidental.
It is likely that one of the reasons the tripel was born was the Vandevelde Act of 1919. This Belgian law, which was not repealed until 1983, forbade the sale or service of strong drink, particularly, Jenever. As neither beer nor wine were affected by this law, it was a commercially opportune time to introduce stronger beers.