10th letter of the English alphabet, pronounced "jay," as in "kay" for -k-, but formerly written out as jy, rhyming with -i- and corresponding to French ji.
One of the most stable English letters (it has almost always the same sound), it is a latecomer to the alphabet and originally had no sound value. The letter itself began as a scribal modification of Roman -i- in continental Medieval Latin. The scribes added a "hook" to small -i-, especially in the final position in a word or roman numeral, to distinguish it from the strokes of other letters. The dot on the -i- (and thus the -j-) and the capitalization of the pronoun I are other solutions to the same problems.
In English, -j- was used as a roman numeral throughout Middle English, but the letter -y- was used to spell words ending an "i" sound, so -j- was not needed to represent a sound. Instead, it was introduced into English c. 1600-1640 to take up the consonantal sound that had evolved from the Roman i- since Late Latin times. In Italian, g- was used to represent this, but in other languages j- took the job. This usage is attested earliest in Spanish, where it was in place before 1600.
No word beginning with J is of Old English derivation. [OED]
English dictionaries did not distinguish words beginning in -i- and -j- until 19c., and -j- formerly was skipped when letters were used to express serial order.
In Latin texts printed in modern times, -j- often is used to represent Latin -i- before -a-, -e-, -o-, -u- in the same syllable, which in Latin was sounded as the consonant in Modern English you, yam, etc., but the custom has been controversial among Latinists:
The character J, j, which represents the letter sound in some school-books, is an invention of the seventeenth century, and is not found in MSS., nor in the best texts of the Latin authors. [Lewis]
In English words from Hebrew, -j- represents yodh, which was equivalent to English consonantal y (hence hallelujah) but many of the Hebrew names later were conformed in sound to the modern -j- (compare Jesus).