An expression that cannot be understood from the meanings of its separate words but must be learned as a whole.

If you had never heard someone say “We're on the same page,” would you have understood that they weren't talking about a book? And the first time someone said he'd “ride shotgun,” did you wonder where the gun was? A modern English-speaker knows thousands of idioms, and uses many every day. Idioms can be completely ordinary (“first off,” “the other day,” “make a point of,” “What's up?”) or more colorful (“asleep at the wheel,” “bite the bullet,” “knuckle sandwich”). A particular type of idiom, called a *phrasal verb* , consists of a verb followed by an adverb or preposition (or sometimes both); in *make over, make out,* and *make up,* for instance, notice how the meanings have nothing to do with the usual meanings of *over, out,* and *up*.

In a manner conforming to the particular forms of a language.

The speech and writing of a native-born English-speaker may seem crude, uneducated, and illiterate, but will almost always be idiomatic—that is, a native speaker always sounds like a native speaker. For a language learner, speaking and writing *idiomatically* in another language is the greatest challenge. Even highly educated foreign learners—professors, scientists, doctors, etc.—rarely succeed in mastering the kind of idiomatic English spoken by an American 7th-grader.

An individual peculiarity of a person's behavior or thinking.

Idiosyncrasies are almost always regarded as harmless. So, for example, filling your house with guns and Nazi posters might be called something stronger than *idiosyncratic*. But if you always arrange your Gummi candies in table form by color and type, then eat them in a special order starting with the pterodactyls (purple ones must die first!), you might qualify. Harmless though your strange habits might be, they may not be the kind of thing you'd tell people about; most Americans are careful to hide their idiosyncrasies, since our culture doesn't seem to value odd behavior. The British, however, are generally fond of their eccentrics, and English villages seem to be filled with them. By the way, few words are harder to spell than *idiosyncrasy*—be careful.

Arising spontaneously or from an obscure or unknown cause.

Words with the *-pathy* suffix generally name a disease or condition (see PATH), so you might think *idiopathic* should describe a disease or condition that's unique to an individual. But the word is actually generally used to describe any medical condition that no one has yet figured out. Most facial tics are called idiopathic by doctors, since no cause can be found. Other well- known conditions, including chronic fatigue syndrome, irritable bowel syndrome, and fibromyalgia, still perplex the medical community. And even though doctors expect that the causes of all of them will eventually be found, and that those causes will turn out to be the same for hundreds of thousands of people, the conditions are still called idiopathic.

(1) Performed in the air. (2) Performed using an airplane.

Shakespeare himself may have coined this word, in *Othello,* and later he gave the name Ariel to the famous air-spirit character in *The Tempest*. An *aerialist* is an acrobat who performs high above the audience. In painting, *aerial perspective* is the way an artist creates the illusion that a mountain or city is far away (something that early painters only slowly learned how to do), usually by making it slightly misty and bluish gray—as if seen through miles of air. An *aerial work platform,* or “cherry picker,” supports a worker at a high elevation on the end of a crane. And *aerial* itself can be used as a noun, meaning a TV antenna, a forward pass in football, or a high-flying stunt performed by a skateboarder or snowboarder.

To supply with air or oxygen.

Faucet *aerators* and aerating showerheads can be easily installed by homeowners to cut water (and especially hot water) use by as much as 50%. A lawn aerator removes little plugs of soil in order to let air deep into the soil, greatly improving the quality of soil that may have gotten too compacted. And a pond aerator, such as a fountain, is a necessity for an ornamental pond with no stream feeding it, since oxygen in the water is necessary to prevent the growth of algae and allow fish to live.

(1) Living or occurring only in the presence of oxygen. (2) Involving or increasing oxygen consumption.

Aerobic exercise is exercise that takes an extended amount of time—usually ten minutes or more—but is usually performed at only moderate intensity. Running, swimming, bicycling, and cross-country skiing are classic aerobic exercises. In 1968 a best-selling book called *Aerobics* introduced a system of exercise for increasing the body's ability to take in and use oxygen, and today aerobics classes, often mimicking such outdoor exercise as running and bicycling, take place every afternoon in thousands of gyms and YMCAs across the country. Aerobic exercise particularly strengthens the heart and lungs, but usually has many other good effects as well. Aerobic bacteria, which need oxygen to live, are essential for breaking down living matter so that it returns to the soil. They include the famous intestinal *E. coli,* as well as the staph and strep bacteria that can make a visit to the hospital risky.

(1) Living or occurring in the absence of oxygen. (2) Relating to activity in which the body works temporarily with inadequate oxygen.

In Greek, the prefix *a-* or *an-* means “not” or “without,” and *bios* means “life.” Anaerobic sports and exercise, such as gymnastics, weight lifting, and sprinting, are of high intensity but short duration, so they don't involve much oxygen intake. Anaerobic exercise triggers a different type of cell activity from aerobic exercise. As a result, it doesn't do much for your heart and lungs and it doesn't burn off fat; what it does do is build muscle. Anaerobic bacteria are bacteria that live without oxygen. They're responsible for several nasty conditions, including tetanus, gangrene, botulism, and food poisoning. They often live in deep wounds, so a bad dog bite—or, even worse, a human bite— can be dangerous, since the mouth is full of anaerobic bacteria. But most anaerobic bacteria are harmless, and many are essential to our lives.