What is the X in X-mas?
"the shedding of light," mid-14c., verbal noun from lighten (v.2). Meaning "alleviation of weight" (literal and figurative) is from 1520s, from lighten (v.1).
"to make less heavy," figuratively "to make cheerful," mid-14c., from light (adj.1) + -en (1). Related: Lightened; lightening.
"shed light upon, illuminate, brighten," early 14c., from light (n.) -en (1). Meaning "to grow brighter" is late 14c. Of faces, expressions, etc., from 1795. Related: Lightened; lightening.
lightening light·en·ing (līt'n-ĭng)n. The sensation of decreased abdominal distention during the latter weeks of pregnancy following the descent of the fetal head into the pelvic inlet.