noun
Pacific oyster
Refers specifically to Crassostrea gigas, a common edible oyster species in Japan.
マガキは冬が旬です。
Pacific oysters are in season in winter.
牡蠣 is the general term for oyster; マガキ specifies the Pacific oyster species.
The origin is uncertain; the kanji 真牡蠣 literally means 'true oyster'.