One who or that which delights; a person or thing that causes delight or brings joy and pleasure to others.
From delight (Old French delite) plus the agent noun suffix -er, creating a noun meaning 'one who delights' or 'something that causes delight.'
The suffix -er (meaning 'one who' or 'thing that') is so productive in English that we can create new words like 'delighter' instantly, even if they're rare—this is why English can name occupations and objects endlessly without needing new dictionary entries.
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