Let’s start from the top: Hyper- is a prefix that means excess or exaggeration, while hypo- is another prefix that means under or beneath. Both hyper and hypo are usually used as prefixes, which are elements or partial words added to the beginning of a base word to modify its meaning. Hyper is derived from the Greek word for over, and hypo is a Greek word that means under. Because they sound very similar, their meanings are often confused.
When you add the prefix hyper- to a word, it indicates an excess or overabundance of some kind. For example, the word hyperactive means extraordinarily active, as in “The student was so hyperactive she couldn’t sit still in class.” Similarly, hyperbole (derived from the Greek word for throwing beyond) means an intentional and obvious exaggeration. An example of hyperbole is the sentence “It took forever for the taxi to arrive.” This is clearly an exaggeration, as forever is an extreme amount of time to wait for a taxi.
In modern usage, the word hyper is also an adjective that means overexcited or overstimulated, as in the sentence “The child was hyper after eating too much candy.” It also indicates a fanatical or obsessive preoccupation, as in “He’s so hyper about germs that he washes his hands 20 times a day.”
Hypo-, is the opposite of hyper-: it means underneath, less than normal, or deficient in some way. For example, the word hypothermia indicates a body temperature that’s far lower than normal. Hypodermic (as in “hypodermic needle”) means under the skin
Hypochondria refers to an excessive preoccupation with and worry about one’s health. The word is derived from the Greek word hypokhondria, which literally translates to under the cartilage (of the breastbone). Its usage dates back to the 16th century when physicians believed the seat of psychosomatic illness was in the upper abdomen. Although doctors no longer believe this, the word retains much of its original meaning.
Science and Medicine
The prefixes hypo- and hyper- are also common in medical and scientific terminology. Hypertension and hypotension, for instance, describe high blood pressure and low blood pressure, respectively. The word hyperbaric refers to greater than normal gaseous pressure, such as in a hyperbaric chamber that delivers high-pressure oxygen. Conversely, the scientific term for deep sea fauna is hypobenthos.
Hyper- and hypo- are easily confused, but their meanings are easier to recall if you associate the terms with an image or idea. For example, to remember the meaning of hyper-, think of a hyperactive friend who is always on the go. Alternatively, remember that your last flu shot was from a hypodermic needle, and the meaning of hypo- becomes very clear.