velocity less than the speed of sound.
Subsonic means "less than the speed of sound."
Slower than the speed of sound. In air, the speed of sound is about 340 m/sec (~1100 ft/sec).
having to do with velocities below the speed of sound (specifically as opposed to ultrasonic, q.v.).
(of speed) less than that of sound in a designated medium; "aircraft flying at subsonic speeds"
a speed less than the speed of sound
Technically, Subsonic refers to "Below the speed of sound". It is also often used in audio specifications to indicate a sound whose frequency is below the limit of human hearing. The proper term to use is Infrasonic.
Flight at speeds below Mach 1.