How do I pass the this context to a function?

I thought this would be something I could easily google, but maybe I’m not asking the right question…

How do I set whatever “this” refers to in a given javascript function?

for example, like with most of jQuery’s functions such as:

$(selector).each(function() {
   //$(this) gives me access to whatever selector we're on
});

How do I write/call my own standalone functions that have an appropriate “this” reference when called? I use jQuery, so if there’s a jQuery-specific way of doing it, that’d be ideal.