助詞「の」. Genitive case particle indicating possession, but can also link general nouns to more specific ones. の can also be a nominaliser.
Genitive case is the grammatical case that marks a noun as modifying another noun. A genitive construction involves two nouns - the head (modified) noun and the modifier noun. The modifier noun modifies the head noun by expressing some property of it.
A nominaliser is usually an affixed morpheme or a particle, that changes another part of speech into a noun or a noun phrase.