"rarer" in the adjective sense
1. rare
not widely known especially valued for its uncommonness
"a rare word"
"rare books"
2. rare
recurring only at long intervals
"a rare appearance"
"total eclipses are rare events"
3. rare
not widely distributed
"rare herbs"
"rare patches of green in the desert"
4. rare, rarefied, rarified
having low density
"rare gasses"
"lightheaded from the rarefied mountain air"
5. rare, uncommon
marked by an uncommon quality especially superlative or extreme of its kind
"what is so rare as a day in June"-J.R.Lowell
"a rare skill"
"an uncommon sense of humor"
"she was kind to an uncommon degree"
6. rare
of meat) cooked a short time still red inside
"rare roast beef"
Source: WordNet® (An amazing lexical database of English)
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