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The Collaborative International Dictionary of English v.0.48 (gcide)

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]