VANC stands for Vertical Ancillary Data Space. This is part of the transmission signal for digital video that is not displayed on the screen as video frames. It is available as a means of carrying data other than the image.
It is described in the SMPTE specification SMPTE-334M "Vertical Ancillary Data Mapping for Bit-Serial Interface". In the PAL and NTSC analog television systems the Vertical blanking interval (or VBI) is similarly used to carry non-display data.