noun, used as a prefix
one large ...; a great ...; a major ...
Prefix used to emphasize scale or importance, often in formal or news contexts. Attaches to nouns to mean 'a single large-scale ...' or 'a major ...'.
その発見は科学界に一大センセーションを巻き起こした。
The discovery caused a great sensation in the scientific community.
彼は一大
He made a major decision and went to study abroad.