In grammar, an intensifier is a word such as 'very' or 'extremely' which you can put in front of an adjective or adverb in order to make its meaning stronger.
英英词典释义
Noun
1. a modifier that has little meaning except to intensify the meaning it modifies;
"`up' in `finished up' is an intensifier""`honestly' in `I honestly don't know' is an intensifier"