noun, noun which may take the genitive case particle 'no'
Used as a noun or with の to mean 'the seventh' in a sequence, such as chapters, ranks, or items.
第七課を復習してください。
Please review Lesson 7.
彼は第七位に入賞した。
He won seventh place.
七番目 is more colloquial and means 'the seventh one' in a countable series, while 第七 is more formal and often used in titles, rankings, or numbered items.
Compound of prefix 第 (ordinal marker) and 七 (seven).