(1) A division into two often contradictory groups. (2) Something with qualities that seem to contradict each other.

In the modern world there's a dichotomy between fast and intense big-city life and the slower and more relaxed life in the country. But the dichotomy is nothing new: the Roman poet Horace was complaining about it in the 1st century B.C. Among other eternal dichotomies, there's the dichotomy between wealth and poverty, between the policies of the leading political parties, between a government's words and its actions—and between what would be most fun to do right this minute and what would be the mature and sensible alternative.

Occurring in two distinguishable forms (as of color or size).

Dimorphism varies greatly in the animal kingdom. Among mammals, the male is generally larger than the female, but other differences in appearance tend to be modest. But birds are usually noticeably dimorphic, with the male being the more colorful sex; when we imagine a pheasant, a mallard, a cardinal, or a peacock, we're almost always picturing the male rather than the female. Among spiders the situation is often reversed. The golden orb-weaver spider, for example, is spectacularly dimorphic: the female may be 20 times the size of the male, and she usually ends up eating him, sometimes even while he's mating with her. Many sea creatures, including many fish, take care of gender problems by simply changing from one sex into the other.

(1) Having two principal elements; double. (2) Allowing electronic communication in two directions at the same time.

*Duplex* can describe a confusing variety of things, depending on the technical field. Most of us use it as a noun: a *duplex* can be either a two-family house or a two-story apartment. In computer science and telecommunications, duplex (or *full-duplex*) communication can go in both directions at once, while *half-duplex* communication can go only one way at a time. In other areas, just translate *duplex* as “double” and see if the sentence makes sense.

Deception by pretending to feel and act one way while acting in another.

The Greek god Zeus often resorted to duplicity to get what he wanted, and most of the time what he wanted was some woman. His duplicity usually involved a disguise: he appeared to Leda as a swan, and to Europa as a bull. Sometimes he had to be *duplicitous* to get around his wife, Hera. After he had had his way with Io and was about to get caught, he turned her into a cow to avoid Hera's anger. ## BI/BIN **BI/BIN** also means “two” or “double.” A *bicycle* has two wheels, and *binoculars* consist of two little telescopes. *Bigamy* is marriage to two people at once. And a road built through the middle of a neighborhood *bisects* it into two pieces.

Involving members of two political parties.

*Partisan* means basically “belonging to a party,” so something bipartisan combines two parties. Since the United States today operates with a two- party system of government (even though the Constitution says nothing about parties at all), legislation often must have some bipartisan support in order to pass into law. Bipartisan committees review legislation, compromising on some points and removing or adding others in order to make the bill more agreeable to both parties and make bipartisan support from the entire legislature or Congress more likely.

(1) Consisting of two things or parts; double. (2) Involving a choice between two alternatives.

*Binary* has many uses, most of them in technical terms. Almost all computer software, for example, is written in *binary code*, which uses only two digits, 0 and 1, 0 standing for a low-voltage impulse (“off”) and 1 standing for a high- voltage impulse (“on”). All information is kept in this form. The word “HELLO,” for example, looks like this: 1001000 1000101 1001100 1001100 1001111.

(1) Occurring every two years. (2) Continuing or lasting over two years.

Biennial conventions, celebrations, competitions, and sports events come every two years. *Biennials* are plants that live two years, bearing flowers and fruit only in the second year. (Carrots and sugar beets are two examples; since we're only interested in their roots, we don't wait another year to see their flower and fruit.) In contrast, *semiannual* means “twice a year.” But no one can agree whether *biweekly* means “twice a week” or “every two weeks,” and whether *bimonthly* means “twice a month” or “every two months.” Maybe we should stop using both of them until we can decide.

Having two opposed forces or views; having two poles or opposed points of attraction.

Magnets are always bipolar: one pole attracts and the other repels or drives away. And the Cold War arms race was bipolar, since it mainly involved the opposing powers of the U.S. and the Soviet Union. But the word is encountered most often today in *bipolar disorder,* the newer name of what used to be called *manic-depressive illness*, in which the person tends to swing between the two extremes, or poles, of high intensity and deep depression, with depression being the main condition. Though an extremely serious illness, bipolar disorder can often be controlled by the drug lithium. ```