The process or act of making something brighter, lighter in color, more luminous, or more cheerful; can also be an adjective describing something that is becoming brighter.
From brighten (Old English beorht + -en) plus -ing, the gerund/present participle suffix. The -ing suffix can create nouns describing the action or adjectives describing ongoing states.
When astrophysicists talk about a star 'brightening,' they're often detecting an explosion, death spiral, or massive collision—what looks beautiful to our eyes is frequently cosmic destruction captured in real-time.
Complete word intelligence in one call. Free tier — 50 lookups/day.