A person overly concerned with his or her own health who often suffers from delusions of physical disease.

One disease a hypochondriac really does suffer from is *hypochondria,* the anxiety and depression that come from worrying too much about one's own health. Even though it's easy to joke about hypochondriacs, hypochondria is no joking matter for the sufferer. Somewhat surprisingly, the second part of *hypochondria* derives from *chondros,* the Greek word for “cartilage.” The cartilage in question is that of the sternum, or breastbone. From ancient times, doctors believed that certain internal organs or regions were the seat of various diseases, both physical and mental, and the area under the breastbone was thought to be the source of hypochondria.

Abnormal decrease of sugar in the blood.

The root *glyk-* means “sweet” in Greek, so *g lyc* shows up in the names of various terms referring to a sugar as a chemical ingredient, such as *glycerine* and *monoglyceride*. People with diabetes have difficulty controlling the sugar in their blood. Too little can be dangerous; its early symptoms may be as minor as nervousness, shaking, and sweating, but it can lead to seizures and unconsciousness. Luckily, it can be taken care of easily by eating or drinking something high in carbohydrates. Its opposite, *hyperglycemia* (see HYPER), is the main symptom of diabetes, and usually requires an injection of insulin, which the sufferer usually gives himself. Today many people—though not doctors—use *hypoglycemia* to mean a completely different condition, with some of the same milder symptoms, that doesn't involve low blood sugar.

Subnormal temperature of the body.

Hypothermia, which usually results from submersion in icy water or prolonged exposure to cold, may constitute a grave medical emergency. It begins to be a concern when body temperature dips below 95°F, and the pulse, breathing, and blood pressure start to decline. Below 90°, the point at which the normal reaction of shivering ceases, emergency treatment is called for.

(1) Involving an assumption made for the sake of argument or for further study or investigation. (2) Imagined for purposes of example.

The noun *hypothesis* comes straight from the Greek word meaning “foundation” or “base”—that is something “put under” something else. So a hypothesis is something you assume to be true in order that you can use it as the base or basis for a line of reasoning—and any such assumption can be called hypothetical. So, for example, the theory that the dinosaurs became extinct because of a giant meteor that struck the earth near the Yucatán Peninsula involves the hypothesis that such a collision would have had such terrible effects on the earth's climate that the great reptiles would have been doomed. Once a hypothesis has been thoroughly studied and researched without being proved wrong, it generally comes to be called a *theory* instead.

(1) Of, relating to, or caused by heat. (2) Designed to insulate in order to retain body heat.

In days gone by, much of the male population of the northern states in the cold months would wear a garment of thermal underwear covering the entire body, called a union suit. Union suits kept sodbusters, cowboys, and townsfolk alike not only warm but also itchy and a little on the smelly side (back when bathing once a week was considered the height of cleanliness). Thermal imaging is photography that captures “heat pictures”—rather than ordinary light pictures—of objects. And thermal pollution occurs when industrial water use ends up warming a river in a damaging way. Small-plane pilots use *thermal* as a noun for a warm updraft, often over a plowed field or desert, that lifts their wings, just as it enables hawks to soar upward without moving their wings.

Physics that deals with the mechanical actions or relations of heat.

Thermodynamics (see DYNAM) is based on the fact that all forms of energy, including heat and mechanical energy, are basically the same. Thus, it deals with the ways in which one form of energy is converted into another, when one of the forms is heat. The study of thermodynamics dates from before the invention of the first practical steam engine—an engine that uses steam to produce physical power—in the 18th century. Today most of the world's electrical power is actually produced by steam engines, and the principal use of thermodynamics is in power production.

*Nuclear* is the adjective for *nucleus*, the main central part of an atom. The original nuclear explosives, detonated in 1945, were so-called *fission* bombs, since they relied on the fission, or splitting, of the nuclei of uranium atoms. But an even greater source of destructive power lay in nuclear *fusion*, the forcing together of atomic nuclei. The light and heat given off by stars such as the sun come from a sustained fusion—or thermonuclear—reaction deep within it. On earth, such thermonuclear reactions were used to develop the hydrogen bomb, a bomb based on a fusion reaction that merged hydrogen atoms to become helium atoms. The thermonuclear era, which began in 1952, produced bombs hundreds of times more powerful than those exploded at the end of World War II. Why the *thermo-* in *thermonuclear*? Because great heat is required to trigger the fusion process, and the trigger used is actually a fission bomb.

Despite its name, the British thermal unit, or BTU, may be more widely used in North America than in Britain. Air conditioners, furnaces, and stoves are generally rated by BTUs. (Though “BTUs” is often short for “BTUs per hour” ; in air-conditioner ratings, for instance, “BTUs” really means “BTUs of cooling capacity per hour.”) Fuels such as natural gas and propane are also compared using BTUs. The BTU first appeared in 1876 and isn't part of the metric system—the metric unit of energy is the much smaller *joule*—so it isn't much used by scientists, but its practicality keeps it popular for consumer goods and fuels. A better-known heat unit is the *calorie*; a BTU is equal to about 252 calories. (Since the familiar food calorie is actually a *kilocalorie*, a BTU equals only about a quarter of a food calorie.)