Chair defined:

A chair is a kind of furniture for sitting, consisting of a back, and sometimes arm rests, commonly for use by one person. Chairs also often have four legs to support the seat raised above the floor. Without back and arm rests it is called a stool. A chair for more than one person is a couch, sofa, settee, loveseat, recliner or bench. A separate footrest for a chair is known as an ottoman, hassock or pouffe. A chair mounted in a vehicle or in a theater is simply called a seat. Chairs as furniture typically can be moved.

Design & ergonomics:
Chair design considers intended usage, ergonomics (how comfortable it is for the occupant), as well as non-ergonomic functional requirements such as size, stack ability, fold ability, weight, durability, stain resistance and artistic design. Intended usage determines the desired seating position. "Task chairs", or any chair intended for people to work at a desk or table, including dining chairs, can only recline very slightly; otherwise the occupant is too far away from the desk or table. Dental chairs are necessarily reclined. Easy chairs for watching television or movies are somewhere in between depending on the height of the screen.

Ergonomic design distributes the weight of the occupant to various parts of the body. A seat that is higher results in dangling feet and increased pressure on the underside of the knees ("popliteal fold"). It may also result in no weight on the feet which means more weight elsewhere. A lower seat may shift too much weight to the "seat bones" ("ischial tuberosities").
http://en.wikipedia.org/wiki/Chair