I just tried to come up with a definition and I feel like I'm almost there but not quite.

A chair is something you can sit on (but you can also sit on a rock)

&& it has a backrest (but the foot of a tree serves as a backrest)

&& it elevates you off the floor (but not all chairs take you above floor level very much)

&& it is manmade.

I believe the correct approach is by looking at purpose, and more importantly primary purpose, rather than try to describe the chair itself.

Eg, a chair is any object whose primary purpose is to sit a single individual

If it's meant to seat multiple people, it's a bench, a sofa, etc, but not a chair

If it's meant to be extra comfortable, it might be a single-seat sofa, but this itself a chair

If it's a rock, but positioned such that it's primary purpose is to sit an individual, such as next to a table, then it's a chair.

And most notably, primary purpose is subjective -- it might be understood as a chair, or as a doorstop, depending on your viewpoint, and what you use it for.

What about stools? I don't consider a stool to be a chair, I consider them two subtypes of seating. If someone told me to grab a chair and gestured to a stool I'd find it very slightly odd.

Fine

To sit an individual at close to, or at, a right angle in regards to their legs

A small rock versus a big rock; Approximating a squat, or standing, is out of bounds

But a stool may consistute a chair when sitting a child

Regardless, I'm mainly talking about the approach to defining it, and that definition by purpose rather than structure is more likely to find a correct definition, not that my example definition is sufficient

> a chair is any object whose primary purpose is to sit a single individual

So is a zabuton[1] a chair?

[1] https://www.dharmacrafts.com/01_zb/zabuton-mat

what if we add: "with one's feet firmly on the ground and legs bent at knee at more or less right angle?"

Highchair.

Now you have to find an accurate and all-capturing definition to all the terms you used (manmade, floor, backrest, ...)

It's a game you cannot win.

Plenty of ergonomic chairs don't have a backrest.

There are also ergo chairs that you "kneel" on instead!

This would classify a Honda Goldwing as a chair, but exclude the Balans kneeling chair.

Also applies to a Sofa

Now I'm doubting if car seats are chairs too...

What is a sofa if not shared chair?

A chair is anything used as a chair. :)

EDIT: Addendum: It may also be worth asking what definition of truth you're using if we're going to be getting into philosophical arguments about what things are: https://en.wikipedia.org/wiki/Truth

And what about an expression like "accounting department chair"?

An accident of the English language; some people would insist on "chairperson" there, and some older people might still insist on "chairman" but that's becoming less and less acceptable over time. As an even better example, there's "fluke":

1. A fish, and a flatworm.

2. The end parts of an anchor.

3. The fins on a whale's tail.

4. A stroke of luck.

The meanings are etymologically distinct (for the most part) and it's only chance they ended up wit the same spelling and pronunciation.

If that's philosophically troublesome, linguistics is not for you.

There’s a bit of leeway here, since that’s the word used in a different sense. But I would say that if you ask someone to “define what a chair is” that the word is being used in one sense only, compared to what happens when you ask someone to “define what ‘chair’ means” which is about the word.

That is, when we refer to a word like “chair” we use quotes, but without the quotes, we can say that an accounting department chair is not a chair.

Alternatively, we could analyze “accounting department chair” as a use of metonymy.

limited namespace conflict? Same word, different abstract concept than the chair under question

I always assumed it came by naming the person who sat in the special seat after the seat itself.

This has a name! It’s called “metonymy.” It’s incredibly common in casual speech and poses unique challenges for natural language processing. https://en.wikipedia.org/wiki/Metonymy

that might be the case but the person himself is not an object to be sat upon (though this may be true metaphorically... hopefully not literally).

“The person that sits in the chair generally used by the head of the accounting department” :)

So the world is… duck typed? :)

That exercise makes me feel like too many people believe that accurate == perfect.