Definition of a "clean install"?

If I buy a refurbed PC with Windows 8.1 and immediately run the upgrade to Windows 10, before putting any software of mine on it, will that be what you'd call a "clean install" of Windows 10?

Then I transfer my stuff from the old Windows 10 upgraded PC to it?

A clean install would be installing Windows from a USB Flash Drive, DVD, or from the computers ownRecovery Partition(ie) set back to factory settings. Eliminating the previous owners personal files and software problems. Then, if you installed Windows 8.1, updating to Windows 10 then restoring your personal files.

First off, in that situation the install of Win10 is no longer available as free. It's possible that a refurbished PC [Desktop or Notebook] will likely have the same software on it as when it was new. A "clean install" means everything is wiped from the drive before installing the Operating System.
Secondly, you can move/copy your "stuff" to wherever but any Windows-based programs would have to be installed, create their icons, add to the Start menu, copy support files to the proper place and create their Registry entries so Windows knows how to run them.

A clean install would be installing Windows from a USB Flash Drive, DVD, or from the computers ownRecovery Partition(ie) set back to factory settings. Eliminating the previous owners personal files and software problems. Then, if you installed Windows 8.1, updating to Windows 10 then restoring your personal files.

This is a factory refurb, which I understand comes with the option to upgrade from 8.1 to 10. So it won't have any software on it to worry about, to begin with. So then I'm thinking of upgrading to 10. THEN it would be pretty much a clean install at that point, right? So I could then copy programs over from her old PC which has had the upgrade to 10 already? Is that how it can work?

Or what you appear to be saying is that I'd have to install the stuff on the new PC after upgrading to Windows 10, then just copy her data over.

First off, in that situation the install of Win10 is no longer available as free.

This still works!

Windows 10 free upgrade is still available using Windows 7 and 8 keys

The Get Windows 10 (GWX) promotions seem to have ended. But I've heard from several Windows users that they've been able to take advantage of the free Windows 10 update using their older Windows 7/8.X product keys after July 29.

Users have been able to both kick off and activate Windows 10 on machines where they've previously installed Windows 10, as well as on machines where they've never installed Windows 10 using their Windows 7/8.X product keys.

This still works!

Windows 10 free upgrade is still available using Windows 7 and 8 keys

It may also work with a factory refurbed PC like this one, but even if it doesn't, it's no big deal.

A clean install would be to upgrade to Windows 10 in order to acquire the license, then download the Windows 10 install media, wipe the hard drive and install Windows 10 from scratch. Upgrading from one Windows version to another is never a clean install.

A clean install would be to upgrade to Windows 10 in order to acquire the license, then download the Windows 10 install media, wipe the hard drive and install Windows 10 from scratch. Upgrading from one Windows version to another is never a clean install.

You do not wipe the hard drive before you clean install W10, with Windows open it is not possible.

Its a bit like trying to remove a chair whilst sitting on it.

The install media removes everything during a clean installation.

Of course you can do that during the installation process. I always remove all partitions or at least format them and start from scratch.