A person or thing that energises; something that gives energy or vitality to something else (British spelling, American 'energizer').
From energise + -er (agent noun suffix, from Old English, meaning 'one who does'). The British -er/-or convention differs slightly from American practices, though both forms exist in both dialects.
The Energizer bunny in American advertising shows how brand names cleverly exploit agent noun suffixes—'energizer' feels like it's doing work, which is exactly what the product promises to do.
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