something that is objectively unnecessary but gives comfort or at least the illusion of comfort

i think of it like the laws of diminishing returns

think of a shitbox $3,000 used car. assuming the engine is more or less running, you get like 80% of the benefits of a car

it gets you from point A -> B - the primary purpose of a car

then you spend another $10,000 for a $13,000 5~6 year old Toyota or something. now you have A/C, that gives you an extra few % benefits. You get a carplay so you have a nice little screen for a GPS, another few %. you get a key that unlocks your car, etc.

so you went from 80% to lets say 90%. but that base 80%, getting you from point A -> B hasn't changed.

that extra $10,000 bought you 10% extra

then let's say you spend another $100,000 for a $113,000 car

you get all the benefits of the previous cars, but you maybe can speed up a little faster. you have heated seats. you have a sport mode or something.

that extra $100,000 bought you another like 7% so now you're at 97%

Luxury is that last 20%. The closer you wanna get to 100%, the more expensive each % costs. This is a status symbol

Necessity, if you don't need it to survive but it makes your feel good it's a luxury, and those things change with time. Today a home Internet connection is nessicary to make it in the world, but 20 years ago it was a luxury and 30 years ago it was a novelty.

Scarcity, something in demand and not shared with others.

Stupid people.

You could do the job with less, but you'd grumble about it.

I think the shower in my current house is luxurious because it's large enough that I can turn around without knocking things off the shower caddy.

I always liked drinking water directly from the tub but in the majority of counties people can't do that so I consider it a luxury. The fact that I live now in a country where it is not possible amplifies that for me even more.

All wares whose demand increases disproportionally with an increasing income.

Bread for example is not a luxury. If you were to earn twice as much, you wouldn't buy much more bread than you already do. The same isn't true for jewelry for example.