A reservist is someone who belongs to the reserve: the extra part of an army, navy, or air force that a country keeps ready, but who is not a full-time soldier in ordinary life. Most of the time they have another job — teacher, driver, student — and they train now and then. If a war or a serious emergency comes, the government can call them up. News and official talk use the word a lot; it is not a chatty kitchen-table word. People sometimes stretch it to anyone kept “on the bench” in a company, though actually it still smells of the military, and that looser use can sound like showing off. The reserve is the group; a reservist is one person in it.