Moving slower than the speed of sound — about 1,235 kilometres an hour in air at sea level. It is an adjective, and you use it mainly about aircraft, bullets and other fast-moving things: a subsonic jet, a subsonic missile. Almost every passenger plane you will ever ride in is subsonic, so in practice the word is used to mark the contrast with the rare things that are not — supersonic ones. That is why you seldom hear it on its own; it turns up where the two are being set against each other, in aviation, military and engineering talk. There is a second, quieter use among shooters and sound engineers: subsonic ammunition is made to fly slower than sound so that it does not make the sharp crack that a supersonic bullet makes. Note also that people sometimes say 'subsonic' when they mean 'sound too low for people to hear' — a very deep bass note. Sound engineers do use it that way, but the careful word for that is 'infrasonic', and a purist will correct you.