(military, law enforcement, chiefly in the plural) An internal rule among a military, police, etc. force defining the circumstances in which and manner with which to use force.

plural of rule of engagement

(transitive) To draw a line underneath, for example to mark the end of a topic or of an accounting period.

(mathematics) The rule that the limit of the ratio of two functions equals the limit of the ratio of their derivatives, usable when the former limit is indeterminate and the latter limit exists.

plural of composing rule

plural of board rule

(informal) The belief that food dropped on the ground may safely be eaten provided it is picked up within five seconds of touching the ground.

(transport, idiomatic) A rule of thumb for safe driving by which a driver must maintain a two-second distance from the vehicle in front.

Usually said in a situation when the rule is incorrect and unusable.

it is acceptable to break rules.