True or in force, but never put into words. An unstated rule is one everybody follows though nobody wrote it down; an unstated assumption is a belief an argument rests on without ever announcing it; an unstated reason is the real reason, left unsaid. The word is useful precisely because it points at a gap: it says the thing exists and the saying of it does not. It is formal and mostly written — you meet it in essays, reports and analysis. Everyday English prefers unspoken (an unspoken rule) or 'nobody said it, but…'. There is a shade of difference worth noticing: unspoken often suggests something felt and shared between people (unspoken love, an unspoken agreement), while unstated is drier and more intellectual, suited to reasons, assumptions, goals and conditions.