A frequency that is below the range of human hearing, conventionally taken to be below 20 Hz. "Subsonic" is not really proper terminology; "infrasonic" is the proper word. But "subsonic" is widely used. The perception of pitch of tones that are near subsonic depends largely on harmonic content; the pitch of a "pure" near-subsonic tone, such as a sine wave, is difficult to perceive. Such tones are a common feature in the hip-hop and drum & bass genres.
The opposite of subsonic is ultrasonic.