lightening / lightning are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
lightening: (noun) descent of the uterus into the pelvic cavity that occurs late in pregnancy; the fetus is said to have dropped.
lightning: (noun) abrupt electric discharge from cloud to cloud or from cloud to earth accompanied by the emission of light. (noun) the flash of light that accompanies an electric discharge in the atmosphere (or something resembling such a flash); can scintillate for a second or more.
lightening: l·AY·t·ah·n·ih·ng
lightning: l·AY·t·n·ih·ng
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)