Where the WebSocket comes from on the client side

I`ve made a socket communication between server-side and the client-side. But where the entity of WebSocket comes from?

Client side code:

var _ws = new WebSocket(...);

That should be comming from somewhere? I mean browser knows the tag comes from HTML, and the console.log('Hi!'); is a JavaScript. But what is the WebSocket?

HTML5?

Browser build in object?