Snake case

Snake case (or snake_case) is the process of writing compound words so that the words are separated with an underscore symbol (_) instead of a space. The first letter is usually changed to lowercase. Some examples of Snake case would be "foo_bar" or "hello_world". It is commonly used in computer code. At least one study found that readers can recognize snake case faster than CamelCase.
Variants.
If the first letter of each word is uppercase, this is known as Train-Case.