noun
large ceremonial mochi
A large round mochi weighing about one shō (approx. 1.8 kg), traditionally used in celebrations such as a child's first birthday (一升餅を背負う) or other festive occasions.
See also: 一升 (いっしょう) · 餅負い
一歳の誕生日に、子どもに一升餅を背負わせる風習があります。
There is a custom of having a one-year-old child carry a large ceremonial mochi on their back on their first birthday.
お祝い用に一升餅を注文した。
I ordered a large ceremonial mochi for the celebration.
鏡餅 is a pair of round mochi used as a New Year decoration, whereas 一升餅 is a single large mochi used in life-event ceremonies like a child's first birthday.
Compound of 一升 (いっしょう, 'one shō', a traditional unit of volume) and 餅 (もち, 'mochi'), referring to a mochi made from one shō of rice.