intensifier
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/ɪnˈtensɪfaɪər/
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/ɪnˈtensɪfaɪə(r)/
A word that makes another word stronger in meaning, like 'so' or 'very'.
The word 'extremely' is an intensifier that makes the adjective 'happy' even stronger in the sentence: 'She was extremely happy about her new job.'