The reduction in vertical definition during vertical image movement due to interlaced (rather than progressive) scans. When interlace is used in broadcast television it means that a field of all the odd lines (1, 3, 5, etc) is sent followed by the even (2, 4, 6, etc.) lines that fit between the odd ones. Experimentally the interlace factor is found to be about 30%. Note that, when scanning film frame-per-frame (i.e. 24 or 25fps, not 3:2 pull-down to 60fps), or a succession of electronic frames each representing a single snapshot in time, there is no vertical movement between fields and so in these cases the Interlace Factor has no effect.
See also: 24PsF