What is the item in the declaration of enum in C#?

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C#

I read a project file and see this declaration. not sure what does @null exactly mean:

public enum ShapeType
   {
       square,
       pyramid,
       staircaseLeft,
       staircaseRight,
       lshapeLeft,
       lshapeRight,
       line,
       @null //<== what does this mean?
   }

What I have tried:

I understand "@" means literal, but does this @null mean?
that mean a literal item?

try to get confirmation from gurus here.