(countable) An understanding between entities to follow a specific course of conduct.

(uncountable) A state whereby several parties share a view or opinion; the state of not contradicting one another.

(uncountable, law) A legally binding contract enforceable in a court of law.

(uncountable, linguistics, grammar) Rules that exist in many languages that force some parts of a sentence to be used or inflected differently depending on certain attributes of other parts.