Bro variables can be complicated to understand because they have a number of possibilities and features. They can be global or local in scope; modifiable or constant (unchangeable); explicitly or implicitly typed; optionally initialized; defined to have additional attributes; and, for global variables, redefined to have a different initialization or different attributes from their first declaration.
Rather than giving the full syntax for variable declarations, which is messy, in the following sections we discuss each of these facets of variables in turn, illustrating them with the minimal necessary syntax. However, keep in mind that the features can be combined as needed in a variable declaration.