suffix
Suffix attached to nouns indicating being under the influence, control, or conditions of something, or during a particular period or event. Common in formal and written contexts.
戦時下で生活は厳しかった。
Life was hard under wartime conditions.
厳しい監視下で作業が続いた。
Work continued under strict surveillance.
From the kanji 下 meaning 'below' or 'under', used as a suffix to express being under a condition or during a period.