What is 'define' used for in JavaScript (aside from the obvious)?

This is AMD pattern for writing modules which AMD stands for Asynchronous Module Definition for when you need to import modules async basically rather than something like commonJS.

define(['module1', 'module2'], function(module1, module2) {
  console.log(module1.sayHi());
});

Define takes an array of dependencies and once all those are loaded in the background (async) in a non-blocking way, define calls the callback which in turn accepts arguments (in this case the dependencies).

Another thing to note is that each one of those modules also needs to be defined using "define" keyword. So for instance module1 would be defined like below :

define([], function() {

  return {
    sayHi: function() {
      console.log('Hi Hi');
    },
  };
});

This way of writing modules (AMD) allows you to write with browser compatibility in mind (no require() like in nodeJS) and also you can define many formats including objects, JSON, etc while for instance commonJS needs modules to be objects.

Keep in mind, AMD has it's own downfalls. Hope this helps someone.

I can't say for sure without seeing the entire script, but it's likely to be the define function from RequireJS, in particular the "define with dependencies" form of that function. It is used to define a "module":

A module is different from a traditional script file in that it defines a well-scoped object that avoids polluting the global namespace. It can explicitly list its dependencies and get a handle on those dependencies without needing to refer to global objects, but instead receive the dependencies as arguments to the function that defines the module.

And the "define with dependencies" form of define is described as follows:

If the module has dependencies, the first argument should be an array of dependency names, and the second argument should be a definition function. The function will be called to define the module once all dependencies have loaded. The function should return an object that defines the module.