What is training frequency?

In one line

Training frequency is how many times you train in a set period, usually a week, counted as training days or as times each muscle was trained.

What is training frequency?
The 'train 3 times a week' or 'chest 2 times a week'
At the gym you often hear 'How many times a week do you train?' or 'I do back 2 times a week'. The number of times here is the training frequency. This post explains how frequency is counted and how it differs from other parts of a workout plan.
In one line
How many times you train in a set period
Training frequency is how many times you train in a set period, usually one week. 'Frequency' means how often the same thing is repeated. So it is usually stated per week, like '3 times a week'.
First way to count: training days
How many days a week you did strength training
The most common way is to count how many days a week you did strength training. The American College of Sports Medicine also defines frequency as 'the number of days per week strength training is performed'. If you trained on Monday, Wednesday, and Friday, that is 3 times a week.
Second way to count: per muscle
How many times a week one muscle was trained
Studies on building muscle count separately how many times a week each muscle was trained. If you trained chest on Monday and Thursday, chest frequency is 2 times a week. It does not matter whether you trained other muscles on other days in between.
When the two counts disagree
Training almost daily, yet each muscle 1 time a week
The two counts often give different numbers. In a split routine, where each day covers only a few body parts, you may train almost every day while each muscle comes around only once a week. In contrast, if you train the whole body every session, the number of training days equals each muscle's frequency.
One part of a workout plan
Alongside weight, rest time, and order
A strength training plan is built from several parts. How heavy you lift, how much you do, how long you rest between sets, and in what order you do exercises, along with frequency, are all parts of it. Each part is set separately when making a plan.
Frequency for health
Strength training on 2 or more days a week
The World Health Organization and the U.S. Department of Health and Human Services advise adults to do strength training that works all the major muscles on 2 or more days a week. These guidelines also count frequency as training days. Not only lifting weights but also body-weight moves like push-ups count as strength training.
Frequency and training volume
'How often' and 'how much' are different
How much you trained, for example the number of sets per week, is called 'training volume'. Whether you do 6 sets of chest once a week or split them into 2 sets three times, weekly sets are the same 6. Frequency decides over how many days the same volume is spread.
Common myth
Does training more often make you stronger by itself?
When studies are pooled, people who trained more often gained more strength. But when only studies that matched weekly training volume are pooled, there was no difference in strength by frequency. Researchers think the extra volume that came with training more often made the difference.
Summary
A word for how many times a week you train
Training frequency is how many times you train in a set period, usually a week, counted as training days or as times each muscle was trained. Gaining more strength when training more often is thought to come from the added training volume rather than from the number of sessions itself. Related terms: training volume, split routine, progressive overload.