An illumination of the intellect, or an inspiration of the will, due partially to the vital activities of these faculties, partly to Divine intervention in the mind's natural process.
Both the illumination and inspiration transcend in intrinsic worth the natural good thoughts and desires of man, by reason of the dignifying influence of God's assisting activity in the eliciting of the thought or desire.
The antecedence in question is relative to the subject's deliberation.
Consequently antecedent grace is the gift of God, preceding, ennobling, and elevating man's mental actions.
New Catholic Dictionary