فالفصاحة في المفرد: خلوصه من تنافر الحروف والغرابة ومخالفة القياس.
Faṣāḥah in a word means freedom from discordant sounds (Tanāfur al-Ḥurūf), unfamiliar obscurity (Gharābah), and irregularity against accepted analogy (Mukhālafat al-Qiyās).
These are different defects: one concerns pronunciation, another intelligibility through usage, and the third a word's morphological formation.
A difficult-to-pronounce word need not be rare; a rare word need not be morphologically irregular. Keeping the three tests separate prevents superficial analysis.