British spelling: a person or thing that harmonises; something that brings elements into harmony or agreement.
From harmonise plus the suffix -r (or -er), indicating an agent noun (something that performs the action). The -er suffix creates nouns meaning 'one who does X' from verbs.
Agent nouns like 'harmoniser' show how English treats verbs and nouns as interchangeable building blocks—almost any verb can become 'one who verbs' by adding -er, which is why English has thousands of occupational nouns derived this way.
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