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

window.length

Returns the number of frames (either frame or iframe elements) in the window.

Example

if (window.length) {
  // this is a document with subframes
}

MDN Reference

Note: No part of this answer comes from me. It all comes from the referred source.