Define j. j synonyms, j pronunciation, j translation, English dictionary definition of j. abbr. 1. Games jack 2. or j joule or J n. pl. j's or J's also js or Js 1. The tenth letter of the modern English alphabet. 2. Any of the speech sounds... J - definition of j by The Free Dictionary.Medical Dictionary‧Legal Dictionary‧Financial Dictionary‧Acronyms‧Idioms‧Encyclopedia.