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(nasm.info)Section 5.4

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5.4. `EXTERN': Importing Symbols from Other Modules
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   `EXTERN' is similar to the MASM directive `EXTRN' and the C keyword
`extern': it is used to declare a symbol which is not defined anywhere
in the module being assembled, but is assumed to be defined in some
other module and needs to be referred to by this one. Not every
object-file format can support external variables: the `bin' format
cannot.

   The `EXTERN' directive takes as many arguments as you like. Each
argument is the name of a symbol:

     extern  _printf
     extern  _sscanf,_fscanf

   Some object-file formats provide extra features to the `EXTERN'
directive. In all cases, the extra features are used by suffixing a
colon to the symbol name followed by object-format specific text. For
example, the `obj' format allows you to declare that the default
segment base of an external should be the group `dgroup' by means of
the directive

     extern  _variable:wrt dgroup

   The primitive form of `EXTERN' differs from the user-level form only
in that it can take only one argument at a time: the support for
multiple arguments is implemented at the preprocessor level.

   You can declare the same variable as `EXTERN' more than once: NASM
will quietly ignore the second and later redeclarations. You can't
declare a variable as `EXTERN' as well as something else, though.