«معرف الشيء ما يحمل عليه لإفادة تصوره»
A thing's definiens is what is predicated of it to bring about its conception.
The definiens is the expression that explains; the definiendum is the thing or concept being explained. In “A human is a rational animal,” “human” is the definiendum and “rational animal” the definiens.
The definition uses predication not to deliver an unrelated fact but to make the subject intelligible.