A substance produced by a living organism (such as bacteria) that is highly poisonous to other organisms.

Long before chemists started creating poisons from scratch, humans were employing natural toxins for killing weeds and insects. For centuries South American tribes have used the toxin curare, extracted from a native vine, to tip their arrows. The garden flower called wolfsbane or monkshood is the source of aconite, an extremely potent toxin. The common flower known as jimsonweed contains the deadly poison scopolamine. And the castor-oil plant yields the almost unbelievably poisonous toxin called ricin. Today we hear health advisers of all kinds talk about ridding the body of toxins; but they're usually pretty vague about which ones they mean, and most of these “toxins” wouldn't be called that by biologists.

The state of being poisonous; the degree to which something is poisonous.

Toxicity is often a relative thing; in the words of a famous old saying, “The dose makes the poison.” Thus, it's possible to die from drinking too much water, and lives have been saved by tiny doses of arsenic. Even though botulinum toxin is the most *toxic* substance known, it's the basic ingredient in Botox, which is injected into the face to get rid of wrinkles. With some poisons, mere skin contact can be lethal; others are lethal when breathed into the lungs in microscopic amounts. To determine if a chemical will be officially called a poison, researchers often use the “LD50” test: If 50 milligrams of the substance for every kilogram of an animal's body weight results in the death of 50% of test animals, the chemical is a poison. But there are problems with such tests, and toxicity remains a very individual concept.

A science that deals with poisons and their effect.

Even though most of us are aware of toxicology primarily from crime shows on TV, *toxicologists* actually do most of their work in other fields. Many are employed by drug companies, others by chemical companies. Many work for the government, making sure the public is being kept safe from environmental poisons in the water, soil, and air, as well as unhealthy substances in our food and drugs. These issues often have to do with quantity; questions about how much of some substance should be considered dangerous, whether in the air or in a soft drink, may be left to toxicologists. But occasionally a toxicology task may be more exciting: for instance, discovering that what looked like an ordinary heart attack was actually brought on by a hypodermic injection of a paralyzing muscle relaxant.

A poisonous protein that acts on the nervous system.

The nervous system is almost all-powerful in the body: all five senses depend on it, as do breathing, digestion, and the heart. So it's an obvious target for poisons, and neurotoxins have developed as weapons in many animals, including snakes, bees, and spiders. Some wasps use a neurotoxin to paralyze their prey so that it can be stored alive to be eaten later. Snake venom is often *neurotoxic* (as in cobras and coral snakes, for example), though it may instead be *hemotoxic* (as in rattlesnakes and coppermouths), operating on the circulatory system. Artificial neurotoxins, called *nerve agents,* have been developed by scientists as means of chemical warfare; luckily, few have ever been used.

Having little substance or strength; flimsy, weak.

Something tenuous has been stretched thin and might break at any time. A person with a tenuous hold on his sanity should be watched carefully. If a business is only *tenuously* surviving, it will probably go bankrupt in the next recession. If there seems to be only a tenuous connection between two crimes, it means the investigators have more work to do.

Thinned or weakened.

A friendship can become attenuated if neither person bothers to keep in touch. Radio waves can become attenuated by the shape of the landscape, by foliage, by atmospheric conditions, and simply by distance. Factory workers and rock musicians often use noise-attenuating ear plugs to save their hearing. To *attenuate* something isn't to stop it, just to tone it down.

Partially excusing or justifying.

*Extenuating* is almost always used today before “circumstances.” Extenuating circumstances are an important concept in the law. If you steal to feed your children, you're naturally less guilty than someone who steals just to get richer; if you kill someone in self-defense, that's obviously an extenuating circumstance that makes your act different from murder. Juries will usually consider extenuating circumstances (even when they're instructed not to), and most judges will listen carefully to an argument about extenuating circumstances as well. And they work outside of the courtroom as well; if you miss your daughter's performance in the middle-school pageant, she may forgive you if it was because you had to race Tigger to the vet's emergency room.

Stretched or bulging out in all directions; swelled.

Before giving you a shot, the nurse may wrap a rubber tube around your upper arm to *distend* the veins. When the heart isn't pumping properly, the skin of the feet and ankles may become distended. A doctor who notices that an internal organ has become distended will always want to find out the cause. As you can see, *distended* tends to be a medical term.