Pacific oyster
Refers specifically to Crassostrea gigas, a common edible oyster species in Japan.
Pacific oysters are in season in winter.
Usually written in katakana; the kanji form 真牡蠣 is rare.
Kanji form is uncommon; the katakana spelling マガキ is standard.
The origin is uncertain; the kanji 真牡蠣 literally means 'true oyster'.