What is a training split (full-body, 2-way split, 5-way split)?

In one line

A training split is a way of training where you divide the body's muscles into groups and take turns training a different group on different days.

What is a training split (full-body, 2-way split, 5-way split)?
The 'leg day' and '5-way split' you hear about
At the gym you hear things like 'today is leg day' or 'I'm on a 5-way split these days'. This is a training split. This post explains what a split is and how full-body, 2-way, and 5-way splits differ.
In short
Divide the body into groups, train a different one each day
A split is a way of training where you divide the body's muscles into a few groups and train a different group on different days. The number in front tells how many groups the body is divided into. Training the whole body at once, without dividing it, is a 'full-body' routine.
Full-body
Train the whole body every session
A full-body routine trains all the large muscles, such as chest, back, and legs, in every session. In English it is also called a 'full-body workout'. Because the whole body is trained each time, each body part gets only a few kinds of exercises.
2-way split
Example: upper-body day and lower-body day
A 2-way split divides the body into two groups. A common example is alternating an upper-body day and a lower-body day. In a study using this method, people trained four times a week: two upper-body days and two lower-body days.
5-way split
Each body part usually comes around once a week
A 5-way split divides the body into five groups, for example chest, back, shoulders, arms, and legs, one per day. So each body part usually comes around once a week. In exchange, that day packs several exercises into one body part.
The word 'frequency'
How many times a week a muscle is trained
How many times a week you train a muscle is called 'frequency'. The number of groups in your split changes this frequency. Full-body 3 times a week means each muscle 3 times a week; a 5-way split done 5 times a week means each muscle 1 time a week.
Why split?
To fit more exercises into each body part
If you train the whole body at once and still give each part many exercises, the session gets too long. Splitting lets you do a lot for one body part while keeping each session a reasonable length. Most bodybuilders who compete also used split routines.
A common misconception
Do you need a split to grow more muscle?
No. When the weekly number of sets per muscle was matched, split and full-body routines showed no difference in muscle or strength gains. The type of split is just a way of dividing the training; it does not set the weekly amount of training (training volume).
Summary
A way of dividing the body, not the same as the amount of training
A split is a way of training where you divide the body's muscles into groups and train a different group on different days. Split or full-body, muscle gains showed no difference when weekly sets were the same. Related terms: training volume, progressive overload, compound and isolation exercises.