tacit
adj.understood without being said or written down. You use this to describe an agreement or a rule that people know exists even if they never talk about it.
adj.understood or implied without being explicitly stated. Often describes a form of agreement, knowledge, or social convention that exists by mutual understanding rather than by formal declaration.
“There is a tacit agreement that we will not talk about politics.”
“The two countries share a tacit understanding that they will not interfere in each other's internal affairs.”
“The board of directors reached a tacit consensus on the new policy, even though no formal vote was ever recorded in the minutes.”
Usage tacit knowledge
“These are forms of tacit learning: knowledge that is difficult to codify and is often acquired through observation, experience and social interaction.”
“At the heart of Denning's argument is the idea of tacit knowledge, the vast amount of human understanding that cannot easily be put into words or represented in a form that computers can process.”
Borrowed from late Middle French tacite, or from Latin tacitus (“that is passed over in silence, done without words, assumed as a matter of course, silent”), from tacere (“to be silent”).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.