To add to other posters, load script is the only built-in option; this is very primitive, but may be enough if your needs are modest.
Late Night Script software The debug editor provides a # include-style library engine that can combine multiple AppleScript files when compiling a script. The disadvantage of Script Debugger is that it buys a couple of hundred dollars, although many regular AppleScript users will tell you that it's worth the investment.
There are several third-party module loaders, Loader and ModuleLoader , which implement more sophisticated import mechanisms on top of the main load script command, and it's worth a look if your requirements are more complex. I did not use ModuleLoader, but the Loader (which I wrote) can import modules at compile time or run time from different standard and user-defined places and automatically resolve complex (even circular) dependencies between modules.
The disadvantages of Loader and ModuleLoader are that they rely on adding scripts to do the hard work, which can be a problem when distributing scripts (in the case of Loader, osax is only needed to compile scripts, not run them), plus you need to add the template code into your Script for actual import.
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