(chemistry) a rule which states that elements that have an even number of protons in the nucleus are more common than those with an odd number

(chemistry) A set of three rules that invoke the conservation of orbital symmetry to predict the stereochemistry of pericyclic reactions.

According to an urban legend, an unofficial guideline used by publishers and the media in the United Kingdom to decide to what degree of erection an image of a penis can be shown.

(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.

A sport created as a mixture of Australian rules football and Gaelic football, to allow international matches between Australia and Ireland.

(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.