فَصْلٌ: الْفَاعِلُ: وَهُوَ كُلُّ اسْمٍ قَبْلَهُ فِعْلٌ أَوْ صِفَةٌ، أُسْنِدَ إِلَيْهِ عَلَى مَعْنَى أَنَّهُ قَامَ بِهِ لَا وَقَعَ عَلَيْهِ.
Translation: The fāʿil is any noun preceded by a verb or a descriptive form, to which the action is attributed in the sense that it proceeds from it, not that the action falls upon it.
Keep four parts of the definition in view: the item is a noun; a verb or descriptive form precedes it; the action is attributed to it; and the definition distinguishes performing an action from receiving it.