A statement which does not perform any action.
The null statement does not perform any action and its only function is to pass on to the next statement. It can be used to indicate that when some conditions are met no action is to be performed. Such an application is useful in particular in conjunction with case statements to exclude some conditions (see example).
case OPCODE is
when "001" => TmpData := RegA and RegB;
when "010" => TmpData := RegA or RegB;
when "100" => TmpData := not RegA;
when others => null;
The example shows an operand detection of a processor, restricted to some simple logical operations performed on registers. All other operations are blocked ("if the OPCODE is other than the explicitly listed, do nothing").
The keyword null is used not only for "no operation" statements. It has a special meaning for variables of access types ("pointing at no object", which is the default value for such variables - see access type and allocator). There is also a null transaction in waveforms. These three applications of the keyword null should not be confused.