reason

Concise Oxford English Dictionary © 2008 Oxford University Press:
reason/ˈriːzn/

▶noun

• 1 a cause, explanation, or justification.

  ■ good or obvious cause to do something: we have reason to celebrate.

  ■ Logic a premise of an argument in support of a belief, especially a minor premise given after the conclusion.

• 2 the power of the mind to think, understand, and form judgements logically.

  ■ (one's reason) one's sanity.

• 3 what is right, practical, or possible.

▶verb

• 1 think, understand, and form judgements logically.

  ■ (reason something out) find a solution to a problem by considering possible options.

• 2 (reason with) persuade with rational argument.

– phrases
by reason of formal because of.
listen to reason be persuaded to act sensibly.
it stands to reason it is obvious or logical.

– derivatives
reasoned adjective,
reasoner noun,
reasonless adjective (archaic).

– origin ME: from OFr. reisun (n.), raisoner (v.), from a var. of L. ratio(n-), from reri ‘consider’.

usage: Many people object to the constructions the reason why … and the reason … is because, and feel that the phrasing the reason that … is more logical and elegant. However, all three expressions are generally accepted in standard English.