oak moss
/ˈəʊk ˌmɒs/noun
Any of certain epiphytic lichens frequently associated with oak trees; specifically those used to form a basis for perfumes, especially Evernia prunastri. Also (frequently in form oakmoss): the perfumed extract from such a lichen.
Origin
Early 18th century; earliest use found in Richard Bradley (?1688–1732), botanist and writer. From oak + moss.