The Collaborative International Dictionary of English v.0.48:

mu \mu\ n. (m[=u])
   The 12th letter of the Greek alphabet ([mu], [MU]).
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

mu
    n 1: the 12th letter of the Greek alphabet

The Jargon File (version 4.4.7, 29 Dec 2003):

mu
 /moo/

    The correct answer to the classic trick question ?Have you stopped beating
    your wife yet??. Assuming that you have no wife or you have never beaten
    your wife, the answer ?yes? is wrong because it implies that you used to
    beat your wife and then stopped, but ?no? is worse because it suggests that
    you have one and are still beating her. According to various Discordians
    and Douglas Hofstadter the correct answer is usually ?mu?, a Japanese word
    alleged to mean ?Your question cannot be answered because it depends on
    incorrect assumptions?. Hackers tend to be sensitive to logical
    inadequacies in language, and many have adopted this suggestion with
    enthusiasm. The word ?mu? is actually from Chinese, meaning ?nothing?; it
    is used in mainstream Japanese in that sense. In Chinese it can also mean ?
    have not? (as in ?I have not done it?), or ?lack of?, which may or may not
    be a definite, complete 'nothing'). Native speakers of Japanese do not
    recognize the Discordian question-denying use, which almost certainly
    derives from overgeneralization of the answer in the following well-known
    Rinzai Zen koan:

        A monk asked Joshu, ?Does a dog have the Buddha nature?? Joshu
        retorted, ?Mu!?

    See also has the X nature, Some AI Koans, and Douglas Hofstadter's G?del,
    Escher, Bach: An Eternal Golden Braid (pointer in the Bibliography in
    Appendix C.


The Free On-line Dictionary of Computing (30 December 2018):

mu

   1.  The country code for Mauritius.

   2.  /moo/ The correct answer to the classic trick
   question "Have you stopped beating your wife yet?".  Assuming
   that you have no wife or you have never beaten your wife, the
   answer "yes" is wrong because it implies that you used to beat
   your wife and then stopped, but "no" is worse because it
   suggests that you have one and are still beating her.
   According to various Discordians and Douglas Hofstadter the
   correct answer is usually "mu", a Japanese word alleged to
   mean "Your question cannot be answered because it depends on
   incorrect assumptions".

   Hackers tend to be sensitive to logical inadequacies in
   language, and many have adopted this suggestion with
   enthusiasm.  The word "mu" is actually from Chinese, meaning
   "nothing"; it is used in mainstream Japanese in that sense,
   but native speakers do not recognise the Discordian
   question-denying use.  It almost certainly derives from
   overgeneralisation of the answer in the following well-known
   Rinzei Zen teaching riddle:

   A monk asked Joshu, "Does a dog have the Buddha nature?"
   Joshu retorted, "Mu!"

   See also has the X nature, AI Koan.

   [Douglas Hofstadter, "Gödel, Escher, Bach: An Eternal Golden
   Braid"].

   [Jargon File]

   (2000-11-22)


The Free On-line Dictionary of Computing (30 December 2018):

Mu

    (Greek letter).

   1.  /micro/ prefix denoting division by 10^6, e.g. mu m
   (micrometre, a millionth part of a metre).  Sometimes written
   as a 'u', the ASCII character nearest in appearance.

   2.  /myoo/ In the theory of functions, mu x . E
   denotes the least value of x for which E = x, i.e. the least
   fixed point of the function \ x . E.  The recursive
   function mu f . H f satisfies (and is defined by) the equation

   	mu f . H f = H (mu f . H f)

   An alternative notation for the same function is

   	fix H = H (fix H)

   See fixed point combinator.

   3.  multiple value.

   [Jargon File]

   (1995-10-30)