What is intermittent fasting?

In one line

Intermittent fasting is a way of eating that sets when to eat, rather than what or how much, and repeats periods of fasting and eating in turn.

What is intermittent fasting?
What does 'I'm doing 16:8' mean?
In talk about dieting, you often hear 'I'm doing 16:8 intermittent fasting these days.' People who fast every other day, or who eat very little on two days a week, use the same name. So what exactly is intermittent fasting?
In one line
Taking turns between fasting time and eating time
Intermittent fasting is a way of eating where you set fasting times and eating times ahead and repeat them in turn. How long the fast lasts depends on the method. Methods range from fasting a few hours a day to fasting a whole day.
What the words mean
'Intermittent' means stopping and starting again
'Intermittent' means happening, pausing and happening again at intervals. 'Fasting' means not eating on purpose for a set time. Put together, it means 'fasting that repeats at intervals'.
How it differs from eating less
It sets 'when' rather than 'how much'
A usual diet focuses on cutting the calories you eat each day. Intermittent fasting focuses on setting when to eat and when to fast. Whether you also cut calories during eating times is decided separately for each method.
Type 1: Time-restricted eating
Eating only within set hours each day
'Time-restricted eating' sets the hours when you eat each day. Usually you eat only within 6-8 hours a day and take in nothing with calories the rest of the time. The same schedule repeats every day.
The numbers in '16:8'
Fast for 16 hours, eat for 8
'16:8' is one form of time-restricted eating: you fast for 16 hours a day and eat only within 8 hours. The first number is the fasting time and the second is the eating time. For example, you skip breakfast and eat only between midday and evening.
Type 2: Alternate-day fasting
Eat one day, fast the next
'Alternate-day fasting' takes turns between a day of normal eating and a fasting day. On fasting days you eat nothing, or eat only 500 kcal or less.
Type 3: The 5:2 diet
Eating very little on just two days a week
It means eating normally five days a week and eating very little on just two days. In a study run by the US National Institute on Aging, the low days were cut to 500-600 kcal.
What happens in the body while fasting
Stored carbohydrate first, then fat
While fasting, the body first draws on 'glycogen', the carbohydrate stored in the liver. When glycogen runs out, the body switches to fat as its main fuel, usually between 12 and 36 hours into a fast. The timing depends on how much you ate before and how much you move during the fast.
Common myth / Can nothing pass your lips during the fast?
What you cut or limit is calories
In one time-restricted eating study, people drank water and calorie-free drinks such as coffee and tea even during fasting hours. Alternate-day fasting also has a form that allows up to 500 kcal on fasting days, and the 5:2 diet still includes a little food on its low days. What intermittent fasting cuts or limits is not 'everything' but 'calories'.
Summary
A way of eating that sets 'when to eat'
Intermittent fasting is a way of eating that sets fasting times and eating times and repeats them in turn. What you cut or limit during the fast is calories, and water and calorie-free drinks were allowed in studies too. Related terms: calorie deficit, glycogen, time-restricted eating.