In the British Army, an NCO is a soldier with rank who leads other soldiers, but who did not take the path that makes a commissioned officer (the kind who start as lieutenants). The letters stand for non-commissioned officer. In practice this usually means ranks such as corporal and sergeant: people who often have years in the job and who run the day-to-day life of a unit, while a young commissioned officer may still be learning. Though actually, soldiers themselves often keep the word “officer” for the commissioned kind, so the tidy dictionary line “officer of low rank” is a little neater than army talk. It is a military word. Civilians mostly meet it in news, history, or films. Other countries have NCOs too, but this sense is the British-army one.