A non-commissioned officer (NCO) is an enlisted leader, petty officer, or in some cases warrant officer, who does not hold a commission. Non-commissioned officers usually earn their position of authority by promotion through the enlisted ranks.
In contrast, commissioned officers usually enter directly from a military academy, officer training corps (OTC) or Reserve Officers' Training Corps (ROTC), or officer candidate school (OCS) or officer training school (OTS), after receiving a post-secondary degree.