The man on the Clapham omnibus is a descriptive formulation of a reasonably educated and intelligent but non-specialist person — a reasonable man; a hypothetical person against whom a defendant's conduct might be judged in an English law civil action for negligence. This standard of care comparable to that which might be exercised by "the man on the Clapham omnibus" was first mentioned by Greer LJ in Hall v. Brooklands Auto-Racing Club (1933) 1 KB 205.

The first reported legal quotation of the phrase is in the case of McQuire v. Western Morning News [1903] 2 KB 100, a libel case, in which Sir Richard Henn Collins MR attributes it to Lord Bowen, who had died nine years earlier.

It is derived from the phrase the bald-headed man at the back of the Clapham Omnibus,[citation needed] coined by the 19th century journalist Walter Bagehot to describe the normal man of London, so used because Clapham in south London at the time was a non-descript commuter suburb and was seen to represent "ordinary" London. Omnibus is a now an archaic expression for a public bus, but would have been common usage amongst the judiciary at the beginning of the 20th century.

This expression has also been incorporated in Canadian patent jurisprudence, notably Beloit v. Valmet OY (1986), C.P.R. (3d) 289 in its eloquent discussion regarding the test for obviousness.

This phrase has arguably been replaced in common parlance by the phrase the man in the street.[citation needed]

In Australia, the "Clapham omnibus" expression inspired a local equivalent, "the man on the Bondi tram"

In Quebec civil law, this expression traditionally translated into "le bon père de famille" to convey the image of the standard prudent man, before being adapted to the more modern "la personne raisonnable" expression.

http://en.wikipedia.org/wiki/The_man_on_the_Clapham_omnibus

The reasonable man or reasonable person standard is often used legal term that originated in the development of the common law. The "reasonable person" is a hypothetical individual who is intended to represent a sort of "average" citizen. The ability of this hypothetical individual to understand matters is consulted in the process of making decisions of law. The question, "How would a reasonable person act under the same or similar circumstances" performs a critical role in legal reasoning in areas such as negligence and contract law.

http://en.wikipedia.org/wiki/Reasonable_person

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