والبلاغة في الكلام: مطابقته لمقتضى الحال مع فصاحته.
Balāghah in speech means conformity with the demands of the situation (Muqtaḍā al-Ḥāl), together with faṣāḥah.
This is a two-part test: linguistic integrity and contextual appropriateness. Elegant wording cannot make up for a statement that is unsuited to its audience or purpose.
A listener who already accepts a point may need only a brief statement; one who doubts or rejects it may need a suitably emphasized explanation.