The act of making something lighter in weight or color, or the visible flash of electricity during a storm (when spelled 'lightning').
From Old English 'leoht' (light) plus the suffix '-ening' indicating a process or action. The word can mean either reducing weight/burden or the quick flashing of light, though the weather phenomenon is typically spelled 'lightning' with an 'n' from 'lightnen' (to lighten).
English spelling is wild here: 'lightening' and 'lightning' are pronounced the same but have totally different meanings, and one misspelling literally erases the 'n' even though the weather word came from 'lighten'! This is a relic of how pronunciation changed but spelling didn't.
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