A variable is a symbolic name paired with a storage location. A variable has a name, a value, and a type. Whatever datum is stored in the location is the value of the variable. This does not include arrays, array elements, records, or record fields, so this definition is more restrictive than the usual usage of the word "variable."
You can specify the type of a variable in a type statement. If the type is not explicitly specified in a type statement, it is implied by the first letter of the variable name: either by the usual default implied typing, or by any implied typing of IMPLICIT statements. See "Types" for more details on the rules for data typing.
At any given time during the execution of a program, a variable is either defined or undefined. If a variable has a predictable value, it is defined; otherwise, it is undefined. A previously defined variable may become undefined, as when a subprogram is exited.
You can define a variable with an assignment statement, an input statement, or a DATA statement. If a variable is assigned a value in a DATA statement, then it is initially defined.
Two variables are associated if each is associated with the same storage location. You can associate variables by use of EQUIVALENCE, COMMON, or MAP statements. Actual and dummy arguments can also associate variables.