Tripel is a term used in the Low Countries to describe a strong ale. The term was notably used in 1956 by the Trappist brewery at Westmalle to rename the strongest beer in their range. A new stronger beer was widely copied by the breweries of Belgium, and in 1987 another Trappist brewery, Koningshoeven in the Netherlands, also expanded their range with La Trappe Tripel. The term is applied by a range of secular brewers to a strong ale in the style of Westmalle Tripel.
The modern origin of tripels lies in Belgium, in the 1930s. Westmalle released a beer under the name Superbier. It was a strong ale and was very likely based on a beer the monks had been brewing sporadically since 1931. In 1956 they renamed it Tripel, and the popularity of that brand ensured the name is still strongly associated with the Westmalle brewery. In 1956, the recipe was modified by Brother Thomas, the head brewer of Westmalle, by the addition of more hops, and it then took on the name Tripel, and it has remained essentially unchanged since.
Tim Webb in his Good Beer Guide to Belgium says that some of the pre-1956 beers called Tripel were dark.