Tripel is a term used originally by brewers in the Low Countries to describe a strong pale ale, and became associated with Westmalle Tripel.
The style of Westmalle's Tripel and the name was widely copied by the breweries of Belgium, then the term spread to the USA and other countries.
The name "Tripel" actually stems from part of the brewing process, in which brewers use up to three times the amount of malt than a standard Trappist "Simple." Traditionally, Tripels are bright yellow to gold in color, which is a shade or two darker than the average Pilsener. Head should be big, dense and creamy. Aroma and flavor runs along complex, spicy phenolic, powdery yeast, fruity/estery with a sweet finish.
Sweetness comes from both the pale malts and the higher alcohol. Bitterness is up there for a beer with such a light body for its strength, but at times is barely perceived amongst the even balance of malts and hops. The lighter body comes from the use of Belgian candy sugar (up to 25% sucrose), which not only lightens the body, but also adds complex alcoholic aromas and flavors. Small amounts of spices are sometimes added as well.
Tripels are actually notoriously alcoholic, yet the best crafted ones hide this character quite evil-like and deceivingly, making them sipping beers.