The Rh blood group system consists of 49 defined blood group antigens.
One of them is the D antigen.
If you test negative for the presence of the D antigen, you are Rh Negative.
This is why Rh Negative is also often listed as Rh(D) negative or D negative.
If you test negative for the presence of ALL antigens of the Rh Blood Group System, you are RHnull.