What is HIIT (high-intensity interval training)?

In one line

HIIT is a way of exercising that repeats short bursts of near all-out effort, each followed by a recovery period of rest or easy movement.

What is HIIT (high-intensity interval training)?
The '1 minute hard, 1 minute easy' workout
At the gym and in workout videos you often hear 'HIIT' or 'intervals'. It means moving very hard for a short time, catching your breath briefly, then going hard again. This post explains exactly what HIIT is and how it differs from similar terms.
In short
Very hard, brief recovery, very hard again
HIIT is a way of exercising that repeats short bursts of near all-out effort, each followed by a recovery period of rest or easy movement. The key point is that hard periods and recovery periods take turns.
The name
'High-intensity' means very hard, 'interval' means a gap
HIIT is short for 'high-intensity interval training'. 'Intensity' means how hard you exercise, and 'interval' means a gap in time. The Korean standard dictionary describes 'interval training' as a practice method that repeats set stretches of fast running and slow running.
How hard?
Too hard to say more than a few words
U.S. health authorities call it 'vigorous' when you cannot say more than a few words without pausing for breath. The hard periods of HIIT are at least this breathless, close to maximal effort. That is why each hard period cannot be kept up for long and ends quickly.
The recovery period
Full rest or easy movement
Recovery periods go between the hard periods. Sometimes you stop completely, and sometimes you walk slowly or pedal lightly to catch your breath.
An example
1 minute hard, 1 minute easy, 10 times
On an indoor bike, you pedal very hard for 1 minute, then pedal lightly for 1 minute to recover. Repeat this 10 times and the hard periods add up to 10 minutes. This is a typical HIIT format often used in research.
Tabata
20 seconds hard, 10 seconds rest, repeated
One well-known type of HIIT is 'Tabata'. You go very hard for 20 seconds and rest for 10 seconds, repeated about 8 times. It was originally a very tough method done on an indoor bike, pushing harder than the effort at which the body uses the most oxygen, until exhaustion.
A harder cousin: sprint intervals
Give everything, then rest a long time
A method even harder than HIIT is 'sprint interval training'. You give everything you have for 30 seconds or less, then rest for about 4 minutes to recover. Researchers separate HIIT, done at near-maximal effort, from sprint intervals, done at effort above the level where the body uses the most oxygen.
Why do it?
It builds heart-lung fitness a lot
The ability to exercise for a long time using the large muscles of the whole body is called 'cardiorespiratory fitness'. Compared with exercising nonstop at moderate effort for a long time, HIIT also raised this fitness a lot, and the gain was slightly larger. The harder sprint intervals raised it about as much as moderate exercise, with far less total exercise.
A common misconception
Is every breathless, nonstop workout HIIT?
No. The backbone of HIIT is that hard periods and recovery periods take turns. Hard workouts that string many different movements together are often compared with HIIT, but they are not the same. Researchers call them 'high-intensity functional training' and separate them from HIIT because recovery periods are not necessarily built in.
Summary
Hard periods and recovery periods take turns
HIIT is a way of exercising that repeats short bursts of near all-out effort with recovery periods in between. Not every breathless workout is HIIT; hard and recovery periods have to take turns. Related terms: aerobic and anaerobic exercise, circuit training, VO2max.