Letter

1. The tenth letter of the basic modern Latin alphabet.

Symbol

1. The symbol for joule, the unit of work or energy in the International System of Units

Letter

1. The tenth letter of the English alphabet, preceded by I and followed by K.

Noun

1. The tenth letter of the English alphabet.
2. A term for a marijuana cigarette ('joint').

   I stepped outside to smoke myself a J. — Paul Simon, from the song "Late in the Evening"

1. Journal
2. Judge or justice, used after a name (plural JJ).

1. 1990: O'Loughlin J — Peabody v Commissioner of Taxation, Federal Court of Australia report http://www.austlii.edu.au/au/cases/cth/federal_ct/unrep5765.html

2. As a prefix, Japanese (J-pop, J-rock, etc.).

Letter

1. The letter J