An armchair is a comfortable, cushioned chair with a support on each side, where you can rest your arms while you sit. Some armchairs are big enough that you can curl up and read a book in them.
Armchairs are distinguished by their "arms," rests built to comfortably support your arms. Most armchairs are also upholstered and soft enough for you to sink into a bit when you sit down. The word is also used as an adjective to jokingly describe someone who isn't an expert in something but acts like one: "Those armchair athletes always seem to know what the players and coaches should have done."