What is the X in X-mas?
1753, from French aperception (17c.), from German Apperzeption (or Latin apperceptionem), coined by German mathematician and philosopher Gottfried Wilhelm Leibniz (1646-1716) as noun corresponding to French apercevoir "perceive, notice, become aware of" (11c., from Latin ad "to" (see ad-) + percipere; see perceive) on analogy of Perzeption/percevoir.
apperception ap·per·cep·tion (āp'ər-sěp'shən)
Conscious perception with full awareness. Also called comprehension.
The process of understanding by which newly observed qualities of an object are related to past experience.