тип, допускающий значение null(In C, an instance of the System.Nullable struct that can represent the normal range of values for its underlying value type, plus an additional null value; In C#, an instance of the System.Nullable struct that can represent the normal range of values for its underlying value type, plus an additional null value); тип, допускающий неопределённое значение(it's a common feature on the .NET runtime, available in all languages Alex_Odeychuk)
