HTML: How do I load a Nav from a separate page?

[JiggyJiggy]

I am creating a site with many pages that will be regularly updated. Instead of creating a nav on each page (and have to update each page when the nav changes) I would like to create 1 nav on a separate page and load it into every page. This way I would only have to make changes to the 1 page, but I am not sure how to tell it to load into each page.