Aka a Graduated Neutral Density, also referred to as a split ND filter, it is like an ND filter except half of it is of neutral density itself and half being optically clear. The demarcation can either be a well defined hard edge or a soft edge. In some cases the entire filter shows a graduation in density , being darkest on one end all the way to lightest on the other end.
ND filters are not demarcated. The entire filter is darkened.
Here is an ND filter
Here are ( 2 types of ) GND filters
ND, GND, and polarisers are some common filters used in photography and there are pretty
much information online as well.