In this context, ADT probably means <i>algebraic</i> data type: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Algebraic_data_type" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Algebraic_data_type</a>. And I don&#x27;t think your definition for abstract data type is right either: usually the language feature that automatically determines types from context is called <i>type inference</i>.