Interstitial spaces are defined by what they are between. They are non-places, between other, proper places. Here is one.

between two walls (Hatton Gallery, Newcastle)

In web design, an interstitial meant a “please wait” screen displayed to deter the user from closing the browser whilst the server was busy – since XmlHttpRequest came along, they’ve disappeared. In architecture, interstitial spaces are found between floors of building, through which ducts and cables can be routed. And in lung physiology, interstitial pneumonia is a disease of the space between the air sacs and their supporting structures, where scarring can prevent oxygen reaching the bloodstream and cause collapsed lungs. It's the reason for COVID-19 patients needing to go on a respirator.

But to call something interstitial, you have to focus on the the things that something can be ‘between’. What if you focus on the thing in the middle, like the web designer whose job it was to design the first please wait screen?

You give it a fancy latin name, I guess, just for a laugh. Only people who look carefully at the URL bar would ever notice, and how many of them would be word geeks who would look it up in the OED and remember it 20 years later? Only me, I would have thought. But if there’s one thing I’ve learnt in life, it’s that nothing is ever only you. There must be other language-geek-web-designers but, as with so many of my interests like Vangelis and Buddhist ethics, they must be out there, I’ve just never met one. So much for the internet making the world smaller, eh?