(adjective) being less dense / (adjective) of, relating to, or interesting to a select group; esoteric / (adjective) very high / (verb) to make rare, thin, porous, or less dense; to expand without the addition of matter / (verb) to make more spiritual, refined, or abstruse / (verb) to become less dense
Wong is able to follow the implications of her research into the more rarefied realms of cultural theory and philosophical aesthetics
it's a nice word whose definition I've never actually looked up