- a separate seat for one person, typically with a back and four legs.
David Attenborough: "Scientists have been studying chairs for several decades, though are only now starting to scrape the surface on what these marvellous creatures really are. Just what a chair actually is, for now, it would seem, will always remain somewhat of a mystery... "
This question does not make sense. Yes, it is something you sit on, but that does not seem to be what is meant here.
If you think about the question as part of learning German, it can make a lot of sense and be useful. A German says to you, "Haben Sie einen Stuhl?" and you think "ACK, oh my goodness what the heck is this guy asking me???" You can then say, thanks to Duolingo ;) "Was ist ein Stuhl?".
This exact question is used in philosophy a bit. See http://www.roangelo.net/logwitt/first-question-philosophy.html#general-definition-of-the-word-chair