In-Band On-Channel digital radio broadcasting

Definition
In-Band On-Channel digital radio broadcasting refers to the method of transmitting a digital radio broadcast signal centered on the same frequency as the AM or FM station's present frequency.

Overview
For FM stations, the transmission of the digital signal occupies the sidebands above and below the center FM frequency (e.g., 97.9 MHz). AM band transmissions also place the digital signal in sidebands above and below the existing AM carrier frequency. By this means, the AM or FM station digital signal is transmitted in addition to the existing analog signal. In both instances, the digital emissions fall within the spectral emission mask of the AM or FM channel (see 47 CFR Sections 73.44 and 73.317).

The present IBOC system is referred to as a "hybrid" since it is neither fully analog nor fully digital. During hybrid operation, existing receivers still continue to receive the analog (non-digital) signal. New receivers will incorporate both modes of reception, where the receiver will automatically switch to the analog signal if the digital signal cannot be decoded or is lost by the receiver.

Source

 * Federal Communications Commission, IBOC Digital Radio Broadcasting for AM and FM Radio Broadcast Stations (full-text).