Coffee.
The reason I answer this is because it is such a spectrum of possible solutions.
Diner coffee. Folger's. Starbuck's.
Then there's whole bean, grinders of various types, machines and preparation methods... and if you go all in you can roast your own beans.
And it's all about relatively subtle differences in the finished product - to the layman.
I can't regularly afford the coffee I prefer but can order from Amazon. I stick to the local grocery store espresso beans for the most part.
And that backs up the prior commenters. I cannot reasonably sustain the habit based on my income. I would love to graduate to 'better' beans but with my budget the money is better used elsewhere.
I am not at a level of stability and comfort where I can get my preferred coffee, let alone get a roaster and beans and do it on my own - which is where the economy of scale kicks in and it gets cheaper again. (I could do it stovetop. Just not a priority enough.)
getting to wake up when you want without impacting anybody else's schedule.
and wearing whatever clothes you want without caring about the arbitrary whims of the social construct of fashion.
in essence, doing things your own way.
Having enough money to be able to cover living expenses including necessary maintenance of the things you own.
The one definitive identifier of financial stability bordering on luxury, to me, is how ever-present paper towel is in any home.
We grew up poor and paper towel was brown stuff that we found in public bathrooms and at gas pumps.
Now we have absorbent and clean paper towel on the counter almost all of the time.
I can tell if the bank is getting near empty, or that there is some big expense coming by the presence, or lack thereof, of paper towels on our counter.
Water to shower with according to Nestle.
Clean drinking water.
Luxury isn't a binary, it's a spectrum based on what's hard to come by and non-essential for you. Early in our marriage, going out for a meal was a luxury, but now we could do it anytime. There have been lots of times when, for me, getting anywhere close to sufficient sleep was a luxury.
If someone doesn't realize something is a luxury, it probably isn't for them.
