This is a general term for conveying a general philosophy, similar to the term REST (Representational State Transfer).Eg: Instead of using events like onclick and onsubmit, the unobtrusive JavaScript attaches to elements by their ID or class based on the HTML5 data- attributes.

Unobtrusive JavaScript is a general approach to the use of JavaScript in web pages. Though the term is not formally defined, its basic principles are generally understood to include: Separation of functionality (the "behavior layer") from a Web page's structure/content and presentation.For more details wiki Link for the same: https://en.wikipedia.org/wiki/Unobtrusive_JavaScript

it is the way by which we can decouple JavaScript logic from presentation layer. eg instead of binding onclick Event inline l, we can use class or id attribute to bind onclick event in JavaScript