고증이 · 고증을
historical research, verification by evidence; period accuracy
Example
이 사극은 시대 고증이 정확하기로 유명해요.
This historical drama is famous for its accurate period detail.
Word parts
고 examine+증 evidence
Literally 'examine and prove' — checking facts against old sources.
Good to know
Common in reviews: 고증이 잘 됐다 / 고증 오류 (historical inaccuracy) for dramas and films.