noun
epoch-making; transition between eras
Refers to a major turning point that marks the beginning of a new era. Often used in the compound 画期的 (かっきてき, 'epoch-making'). As a standalone noun, it is somewhat literary.
See also: 画期的
この発明は科学の画期をなすものだ。
This invention marks an epoch in science.
明治維新は日本の歴史における大きな画期だった。
The Meiji Restoration was a major turning point in Japanese history.
The common adjective form meaning 'epoch-making' or 'groundbreaking'. 画期 is the noun root, used less frequently on its own.
From 画 (かく, 'divide') + 期 (き, 'period'). The kanji 画 originally meant 'to draw a line', hence 'to demarcate', giving the sense of dividing historical periods.