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.