حَقِيقَةُ الشَّيْءِ وَمَاهِيَّتُهُ
The reality of a thing and its quiddity, or what-it-is.
The question “What is it?” seeks a thing's kind or defining nature. Māhiyyah is linked to the question mā huwa? (“What is it?”).
Understanding what a kind of thing would be does not by itself establish that an instance exists outside the mind. Defining a nature and affirming its external instantiation are distinct operations.