الفَصَاحَةُ: هِيَ فِي الْأَصْلِ تُنْبِئُ عَنِ الْإِبَانَةِ وَالظُّهُورِ
Originally, fasaha conveys the senses of making clear and becoming manifest.
Ibana suggests making an intended meaning explicit; zuhur suggests manifestness as opposed to concealment. These are the linguistic roots of the discussion, not yet the complete technical definition.
Simply understanding a sentence does not prove it satisfies every condition of classical fasaha. In technical analysis, diction, established usage, and other features also matter.