Definition of rare adjective from the Oxford Advanced Learner's Dictionary
rare
adjective
reə(r) rer
rarer, rarest
1not done, seen, happening, etc. very oftena rare disease/occurrence/sightThis weekend, visitors will get a rare chance to visit the private apartments.rare (for somebody/something to do something)It's extremely rare for it to be this hot in April.rare (to do something)It is rare to find such loyalty these days.On the rare occasions when they met he hardly even dared speak to her.It was a rare (= very great) honour to be made a fellow of the college.2existing only in small numbers and therefore valuable or interestinga rare book/coin/stampa rare breed/plantThis species is extremely rare.3(of meat) cooked for only a short time so that the inside is still red‘How would you like your steak?’ ‘Rare.’ compare well done see also rarity
reə(r) 
rer 
1 not done, seen, happening, etc. very oftena rare disease/occurrence/sightThis weekend, visitors will get a rare chance to visit the private apartments.rare (for somebody/something to do something) It's extremely rare for it to be this hot in April.rare (to do something) It is rare to find such loyalty these days.On the rare occasions when they met he hardly even dared speak to her.It was a rare (= very great) honour to be made a fellow of the college.