Tacit
Etymology
From Latin "tacitus," "silent," the past participle of "tacēre" ("to be silent"). The same root gives us taciturn and, more distantly, reticent. English kept the core idea intact: something tacit is conveyed by silence, implied rather than spoken.
adjective · /TAS-it/
Understood or agreed without being put into words.
What it names
What a group knows but never says aloud — a tacit rule, a tacit approval — where the silence is itself the medium of agreement.
Example
Her raised eyebrow was tacit permission to keep going.
Related idea
Compare implicit: both mean unstated, but implicit points to what is logically contained in something, while tacit points to what is knowingly left unsaid.