Unobtrusive JavaScript is a best practice methodology for attaching JavaScript to the front-end of a website. It's an ideal to strive toward and something we should bear in mind whenever we're adding JavaScript to a site.

It is the separation of behaviour from markup or structure. Just like the CSS gurus of old taught us there should be a separation of layout from markup, there should be a separation of behaviour from markup. That's HTML for the content and structure of the document, CSS for the layout and style, and Unobtrusive JavaScript for behaviour and interactivity.