What is the global length variable (window.length)?

Question

I have just observed that every page has length global variable.

What does this variable represent? Is it somehow related to how many iframes/frames are

Answer 1

Yes, it's how many frames (including iframes) are in the current window:

https://developer.mozilla.org/en-US/docs/Web/API/Window.length

W3 specification:

http://www.w3.org/html/wg/drafts/html/master/browsers.html#accessing-other-browsing-contexts