j function

from The Century Dictionary.

• The tenth letter in the English alphabet.
• As a numeral, a variant form of I: used chiefly at the end of a series of numerals, and now only in medical prescriptions: as, vj (six); viij (eight).
• In mathematics, -j stands for the second unit vector or other unit of a multiple algebra. J usually denotes the Jacobian.
• In thermodynamics, J is the mechanical equivalent of heat (being the initial of Joule).
• See function.