(also called Form).— 1. A principle of act.

Accidental Form.— A principle of act in a substance, whether a body or a spiritual substance, that does not belong directly to the nature of the substance and determines the nature of an accident of the substance. 

Substantial Form.— A principle of act in a bodily being that, together with primary matter, constitutes a body as such and determines the nature of the body.  In the case of the angels, the immediate subject of the act of existence. 

Form of the Part (in Latin, Forma Partis).— Substantial form. 

Form of the Whole (in Latin, Forma Totius).— Essence. 

2. Form can also be used as synonym for “essence,” thereby focusing on the essence’s informing of matter.