What does a double colon followed by an equals sign (::=) mean in programming documentation?

Question

What does ::= mean in programming documentation?
For example in the Lua documentation: or in Python documentation.

Answer 1

This is Backus-Naur Form (BNF) notation describing the language. ::= in this context means is defined as.

For example, in the Python language documentation you refer to, an identifier is defined as a letter or an underscore, followed by a letter, a digit or an underscore. The notation then goes on to describe what a letter and a digit is defined as, and so on.