(1) A group of three usually related people or things. (2) A secret Chinese criminal organization.

The best-known type of triad is a type of musical chord consisting of three notes. A D-major triad is made up of the notes D, F-sharp, and A; an F-minor triad is made up of F, A-flat, and C; and so on. Major and minor triads form the basis of tonal music, and songs and other pieces usually end with a *triadic* harmony. In medicine, a triad is a set of three symptoms that go together. The Chinese criminal organizations called triads got their name from the triangular symbol that they used back when they began, centuries ago, as patriotic organizations. Today, with over 100,000 members, the triads operate in the U.S., Canada, and many other countries.

A series of three creative works that are closely related and develop a single theme.

Dozens of tragic trilogies were written for the Greek stage, though only one, Aeschylus's great *Oresteia* (consisting of *Agamemnon, The Libation Bearers,* and *The Eumenides*), has survived complete. Authors in later years have occasionally chosen to create trilogies to allow themselves to develop a highly complex story or cover a long span of time. Tolkien's *Lord of the Rings* wasn't actually intended as a trilogy, but since it was published in three volumes it's usually called one. George Lucas's three original *Star Wars* movies are an example of a film trilogy (which he followed many years later by another).

The name *triceratops,* meaning literally “three-horned face,” refers to the two horns above its eyes and the smaller third horn on its snout. Just as striking was the frilled hood or ruff that rose behind its head, though no one is quite sure what it was for. The triceratops was one of the last dinosaurs to evolve and also one of the last to become extinct. It could reach lengths of 30 feet and could stand nearly eight feet high. Despite its ferocious looks and three- foot-long horns, the triceratops was actually a vegetarian.

A three-pronged spear, especially one carried by various sea gods in classical mythology.

A trident has three prongs or teeth, as the root *dent,* “tooth,” tells us. The trident has long been used to spear fish in different parts of the world, so there's no mystery about why the Greek sea god Poseidon and his Roman counterpart Neptune both carry a trident as their symbol. In some gladiator exhibitions in ancient Rome, one gladiator, called a *retiarius* (“net man”), would be equipped as though he were a fisherman, with a weighted net and a trident; with his net he would snare his sword-wielding opponent, and with his trident he would spear his helpless foe.

(1) A period of about three months, especially one of three such periods in a human pregnancy. (2) One of three terms into which an academic year is sometimes divided.

*Semester,* which comes from the Latin words for “six” and “month,” has come to mean half an academic year when the year is divided into two segments. When an academic year is divided into three segments, each is called a trimester (which is usually a bit more accurate, since each segment often is close to three months in length). Some colleges operate on the “quarter” system, with the summer being the fourth quarter, but this just means that each quarter is basically a trimester. In a human pregnancy, a trimester is three months long, representing one-third of the nine months that a typical pregnancy lasts.

(1) (capitalized) The unity of Father, Son, and Holy Spirit as three persons in one God in Christian belief. (2) A group of three people.

The nature of the Trinity (or Holy Trinity) has caused centuries of argument and division within the Christian faith. The word doesn't actually appear in the Bible itself, but the New Testament does speak of the Father, Son, and Holy Spirit together; the Father is understood as the protector of the Jews, the Son as the savior of mankind, and the Holy Spirit as the preserver of the church. Almost all the major Christian sects may be called *trinitarian.* The island of Trinidad is one of many places named with the Spanish translation of *Trinity*.

(1) A picture or carving made in the form of three panels side by side. (2) Something composed or presented in three sections.

*Triptych* contains the root *-ptyche,* the Greek word for “fold.” So a traditional painted or carved triptych has three hinged panels, and the two outer panels fold in toward the central one. Most triptychs were intended to be mounted over a church altar. Many great triptychs were produced in the Renaissance, perhaps the most famous being Hieronymus Bosch's *Garden of Earthly Delights*. But major triptychs continued to be produced throughout the 20th century by such painters as Francis Bacon.

Of little value or importance.

*Trivial* comes from a Latin word meaning “crossroads”—that is, where three roads come together. Since a crossroads is a very public place where all kinds of people might show up, *trivialis* came to mean “commonplace” or “vulgar.” Today the English word has changed slightly in meaning and instead usually describes something barely worth mentioning. Mathematicians use the word to refer to some part of a proof or definition that's extremely simple and needn't be explained, but the rest of us tend to use it just to mean “unimportant.” “ Small talk” at a party, for example, is usually trivial conversation, though a trivial excuse for not going on a date (“I have to wash my hair”) might hide an emotion that isn't so trivial (“I can't stand the sight of you”). To *trivialize* something is to treat it as if it didn't matter, as if it were just another *triviality.* ```