The Collaborative International Dictionary of English v.0.48:

J \J\ (j[=a]).
   J is the tenth letter of the English alphabet. It is a later
   variant form of the Roman letter I, used to express a
   consonantal sound, that is, originally, the sound of English
   y in yet. The forms J and I have, until a recent time, been
   classed together, and they have been used interchangeably.
   [1913 Webster]

   Note: In medical prescriptions j is still used in place of i
         at the end of a number, as a Roman numeral; as, vj,
         xij. J is etymologically most closely related to i, y,
         g; as in jot, iota; jest, gesture; join, jugular, yoke.
         See I. J is a compound vocal consonant, nearly
         equivalent in sound to dzh. It is exactly the same as g
         in gem. See Guide to Pronunciation, [sect][sect] 179,
         211, 239.
         [1913 Webster]

WordNet (r) 3.0 (2006):

J
    n 1: a unit of electrical energy equal to the work done when a
         current of one ampere passes through a resistance of one
         ohm for one second [syn: joule, J, watt second]
    2: the 10th letter of the Roman alphabet [syn: J, j]

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

J

   A derivative and redesign of APL with added features and
   control structures.  J is purely functional with lexical
   scope and more conventional control structures, plus several
   new concepts such as function rank and function arrays.  J
   was designed and developed by Kennneth E. Iverson and Roger
   Hui .  J uses only the ASCII
   character set but has a spelling scheme that retains the
   advantages of APL's special alphabet.  J is a conventional
   procedural programming language but can be used as a purely
   functional language.

   Version 4.1 for MS-DOS, Sun, Mac, Archimedes.  Source
   available in C from Iverson Software, +1 (416) 925 6096.

   Version 6 package from ISI includes an interpreter and
   tutorial.  Ported to DEC, NeXT, SGI, Sun-3, Sun-4,
   Vax, RS/6000, MIPS, Macintosh, Acorn Archimedes,
   IBM PC, Atari, 3b1, Amiga.

   (ftp://watserv1.waterloo.edu/languages/apl/j).

   J-mode GNU Emacs macros available by
   (ftp://think.com/pub/j/gmacs/j-interaction-mode.el).

   ["APL\?", Roger K.W. Hui et al, APL90 Conf Proc, Quote Quad
   20(4):192-200].

   (1992-10-31)


The Devil's Dictionary (1881-1906):

J is a consonant in English, but some nations use it as a vowel --
than which nothing could be more absurd.  Its original form, which has
been but slightly modified, was that of the tail of a subdued dog, and
it was not a letter but a character, standing for a Latin verb,
_jacere_, "to throw," because when a stone is thrown at a dog the
dog's tail assumes that shape.  This is the origin of the letter, as
expounded by the renowned Dr. Jocolpus Bumer, of the University of
Belgrade, who established his conclusions on the subject in a work of
three quarto volumes and committed suicide on being reminded that the
j in the Roman alphabet had originally no curl.