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

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

A variety of football devised in Australia.

(chiefly US) The notion that one drop of black blood (i.e., any African ancestry at all) makes a person black.

(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

(law) The obligation of a barrister (in certain jurisdictions) to accept any work in a field in which they profess themselves competent to practise, at a court at which they normally appear, and at their usual rates.

(soccer) The prohibition of teammates closer to the opponents' goal than the person kicking or throwing the ball unless there are three opponents at least as close to their goal line.

(idiomatic) Examine carefully and thoroughly.

plural of white coat rule

(advertising) A rule prohibiting the use of actors dressed as medical professionals to give a misleading impression of the medical benefits of a product being advertised.