"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).
Entries linking to lightening
lighten v.2
"shed light upon, illuminate, make light or bright," early 14c., from light (n.) + -en (1). Intransitive meaning "to become brighter" is late 14c.; of faces, expressions, etc., from 1795. Meaning "to flash lightning" is from mid-15c. Related: Lightened; lightening.
lighten v.1
"to make less heavy, to lighten a load," mid-14c., lighten, lightnen, from light (adj.1) + -en (1). Figuratively "to make cheerful" from c. 1400. Intransitive sense "become less heavy" is from 1720. Related: Lightened; lightening.