Tachy- is a combining form meaning “fast” or “rapid,” taken from the Ancient Greek word tachys, “swift.” You’ll see it most in medicine, where tachycardia means a fast heart rate. It also turns up in physics, geology, zoology and old shorthand systems.
What tachy- means
Put tachy- in front of a base word and you get “fast [base].” Tachycardia is a fast heart, tachypnea is fast breathing, and tachygraphy is fast writing. Every word built on it follows that pattern, which makes it one of the easier prefixes to learn.
In dictionary terms, tachy- is a combining form rather than a true standalone word. It needs a second element to attach to, usually another Greek root such as kardia (heart), pnoia (breath) or graphia (writing). That Greek pairing is why so many tachy- words sound technical. Doctors and scientists coined most of them by combining Greek parts on purpose, mainly in the 19th and 20th centuries.
How to pronounce tachy-
Say it TAK-ee, IPA /ˈtæki/. The “ch” is a hard K sound, as in chorus or character. That’s the standard treatment for words that came into English from Greek through Latin.
Stress usually shifts once the word grows:
- tachycardia: tak-ih-KAR-dee-uh
- tachypnea: tak-IP-nee-uh (or tak-ip-NEE-uh)
- tachyon: TAK-ee-on
- tachyphylaxis: tak-ee-fih-LAK-sis
Usage tip
Never say tachy- with a soft “ch” like chair. “TATCH-ee-cardia” is a common slip among new nursing and medical students, and listeners will notice it.
Where tachy- comes from
The source is Ancient Greek ταχύς (tachys), an adjective meaning “swift” or “quick.” The same Greek family gave English two close cousins, and recognising them helps you read unfamiliar terms:
| Greek word | Meaning | English form | Example |
|---|---|---|---|
| ταχύς (tachys) | swift | tachy- | tachycardia |
| τάχος (tachos) | speed | tacho- | tachometer |
| τάχιστος (tachistos) | swiftest | tachisto- | tachistoscope |
So a tachometer measures speed (tacho-), while tachycardia describes a heart that is fast (tachy-). The meanings overlap. The spelling tells you which Greek word the coiner had in mind.
15 common words built on tachy- (and its cousins)
Here is the working list you’re most likely to meet, grouped by field. The last few use the tacho- and tachisto- forms, which share the same root.
| Word | Meaning | Field |
|---|---|---|
| tachycardia | abnormally fast heart rate | medicine |
| tachypnea | abnormally fast breathing | medicine |
| tachyarrhythmia | fast heart rate with an irregular rhythm | medicine |
| tachyphylaxis | rapidly decreasing response to a repeated drug dose | pharmacology |
| tachykinin | a family of neuropeptides that trigger quick muscle contraction | biology |
| tachyzoite | the fast-multiplying stage of parasites like Toxoplasma | microbiology |
| tachylalia | very rapid speech | speech pathology |
| tachyphagia | eating very fast | medicine |
| tachygraphy | shorthand; fast writing | history, writing |
| tachyon | a hypothetical particle that always travels faster than light | physics |
| tachylite | a dark volcanic glass | geology |
| tachymeter | an instrument or watch scale for measuring speed or distance quickly | surveying, watches |
| Tachyglossus | the genus of the short-beaked echidna (“swift tongue”) | zoology |
| tachometer | a gauge showing engine speed in revolutions per minute | engineering |
| tachistoscope | a device that flashes images for a split second | psychology |
Medical words
This is where most people first meet tachy-. Tachycardia in adults usually means a resting heart rate above 100 beats per minute. Tachypnea means breathing faster than normal; for a resting adult, that is generally above about 20 breaths a minute.
Tachyarrhythmia combines tachy- with arrhythmia (an irregular rhythm). It covers fast rhythm problems such as atrial fibrillation with a rapid rate. You’ll also see tachy-brady syndrome, a form of sick sinus syndrome in which the heart swings between too fast and too slow.
Tachyphylaxis pairs tachy- with Greek phylaxis, “protection.” It describes a drug losing its effect quickly after repeated doses. Some decongestant nasal sprays are a familiar example. You can’t fix it simply by taking more.
British medical writing often spells these words differently. Tachypnea becomes tachypnoea, keeping the Greek-derived “oe.” It follows the same habit that makes litre the British spelling of liter.
Science and technology words
Tachyon has a precise origin. Physicist Gerald Feinberg coined it in a 1967 paper on the possibility of faster-than-light particles. No tachyon has ever been detected. Science fiction loves the word anyway, and Star Trek scripts used “tachyon” freely.
Tachylite (also spelled tachylyte) is a basaltic volcanic glass. The name joins tachy- with Greek lytos, “able to be dissolved,” because the rock breaks down or melts readily.
Tachymeter has two lives. Surveyors use the name for instruments that measure distance quickly. Watch collectors know it as the scale printed around a chronograph bezel. Time a car over one kilometre and the scale reads off your average speed.
Names from nature
Biologists use tachy- in scientific names for fast animals and fast body parts. Tachyglossus, the short-beaked echidna, means “swift tongue,” which fits an animal that flicks its tongue into ant nests. The little grebe’s genus, Tachybaptus, means roughly “fast diver.”
“Tachy” as clinical slang
On hospital wards, “tachy” often works as a standalone adjective. A nurse might say, “Bed 4 is tachy, 118 and climbing.” It’s short for tachycardic and sounds informal but is completely standard in spoken clinical English.
A few examples of how staff use it:
- “She’s been tachy since the fever spiked.”
- “He went tachy after the second dose, so we held it.”
- “Sinus tachy, no ectopics.” (sinus tachycardia on an ECG)
Keep it for speech and quick notes among colleagues. Formal charting and patient letters should spell out tachycardia or tachycardic. Car enthusiasts have a parallel clipping: “tach,” short for tachometer, as in “watch the tach when you shift.”
Tachy- vs brady-: the opposite prefix
The partner you need to know is brady-, from Greek bradys, “slow.” Medicine pairs the two constantly:
| Fast (tachy-) | Slow (brady-) |
|---|---|
| tachycardia: heart rate over 100 | bradycardia: heart rate under 60 |
| tachypnea: fast breathing | bradypnea: slow breathing |
| tachykinin: fast-acting peptide | bradykinin: slower-acting peptide |
| tachylalia: rapid speech | bradylalia: slow speech |
The tachykinin and bradykinin pair is a neat case. Researchers named tachykinins for making gut muscle contract more quickly than bradykinin does. Learn one prefix and you’ve effectively learned both halves of the vocabulary.
Often confused with
Tacho- (tachometer, tachograph) comes from tachos, “speed,” and names devices that measure it. Tachy- describes something that is fast. Tachycardia is a fast heart; a “tachocardia” doesn’t exist.
How to decode a tachy- word you’ve never seen
Medical and scientific terms look hard until you split them. Use this three-step routine:
- Strip off tachy- and replace it with “fast.”
- Identify the base. Greek roots are common: -cardia (heart), -pnea (breathing), -phagia (eating), -lalia (speech), -graphy (writing).
- Check the ending. -ia usually signals a condition, -meter a measuring device, and -on a particle in physics.
Try it on tachyphagia. Tachy- means fast, -phagia means eating, and -ia marks a condition. The result is “the condition of eating fast,” which is exactly what the word means. Once you know the ins and outs of a handful of Greek roots, most of these terms explain themselves.
Tachy- in brief
Tachy- means fast, comes from Greek tachys, and is pronounced TAK-ee. It sits at the front of everyday clinical words like tachycardia and tachypnea. It also appears in rarer coinages like tachyon and tachyphylaxis. Remember its opposite, brady-, and its spelling cousins, tacho- and tachisto-, and you can read most of the family on sight.
FAQ
What does tachy mean in medical terms?
It means fast or rapid. In medical terminology, tachy- most often describes a body function running faster than normal, as in tachycardia (fast heart rate) or tachypnea (fast breathing). Used alone in speech, “tachy” is clinical shorthand for tachycardic.
Is tachy- Greek or Latin?
Greek. It comes from tachys, meaning “swift.” Many tachy- words reached English through modern scientific Latin. The root itself is Ancient Greek, which explains the hard K pronunciation of the “ch.”
What is the opposite of tachy-?
Brady-, from Greek bradys, “slow.” Tachycardia and bradycardia, or tachypnea and bradypnea, are the classic opposite pairs.
Is tachometer a tachy- word?
Not exactly. Tachometer uses tacho-, from Greek tachos (“speed”), a close relative of tachys. Both forms carry the idea of speed. The tacho- form tends to appear in names for measuring devices, such as tachometer and tachograph.
Is tachycardia always dangerous?
No. Your heart rate rises above 100 during exercise, fever, fear or after strong coffee, and that sinus tachycardia is a normal response. A fast rate at rest, or one paired with chest pain, fainting or breathlessness, needs prompt medical attention.
About this article
- Fact-checked by Maureen Talbot before publication.
- Published