(idiomatic) A clear-cut, easy to make a decision. / (law) A clearly defined rule or standard, composed of objective factors, which leaves little or no room for varying interpretation.

The rare occurrence of a counterexample to a rule, used to demonstrate that the rule exists.

(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.

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

(baseball) A rule providing that a fair fly ball hit with a force play at third base, which is deemed catchable by an umpire, be ruled an out irrespective of the fielder's play, eliminating the force play at all bases.

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

(law, investment) A standard for the duty of a fiduciary with responsibility over investments.

a rule of thumb by which a driver may maintain a safe following distance, allowing at least a two second interval behind any vehicle that is directly in front of the driver's vehicle.

(law, singular only) The rule that prevents a testator or other transferor of property from controlling further transfer of his property more than twenty-one years after the death of anyone alive at the time of the original transfer who may have some interest in the transfer.

A common mnemonic for understanding notation conventions for vectors in three dimensions. / (logic) The systematic use of right-hand turnings to solve a maze.