adverb
an adverb modifies or describes a verb, adjective, or another adverb; in this case, 'rarely' modifies the frequency of an action or event, indicating that it happens infrequently or seldom
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In academic circles, it is rarely acceptable to use anecdotal evidence in research papers.
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I rarely go to the gym because I prefer exercising outdoors.
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I rarely hit the snooze button in the morning because I'm a morning person.
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True love is like finding a rare gem, it rarely comes around more than once in a lifetime.