just cause; good reason; pretext; justification
Refers to a morally or publicly acceptable reason for an action, often used in political, military, or organizational contexts. Can carry a nuance of a pretext or excuse when the stated reason is not the real motive.
戦争を始めるには大義名分が必要だ。
A just cause is necessary to start a war.
彼は改革の大義名分として経済危機を挙げた。
He cited the economic crisis as justification for the reforms.
duty to one's country or sovereign
A more traditional, somewhat archaic sense referring to a moral obligation to serve one's country or ruler. Rare in modern everyday speech but still understood in historical or formal contexts.
武士にとって主君への大義名分は絶対だった。
For samurai, duty to their lord was absolute.
The company carried out layoffs under the banner of business restructuring as its justification.
Troops were dispatched under the pretext of maintaining peace.
This gave us a solid justification for opposing it.
That statement gave him the justification he needed to attack.
名目 is a nominal or stated reason that may or may not be sincere, while 大義名分 specifically frames that reason as a lofty, principled cause.