J J /ˈdʒeɪ/

noun

1. The 10th letter of the Roman alphabet  ( J )
   the 10th letter of the Roman alphabet
   
2. A unit of electrical energy equal to the work done when a current of one ampere passes through a resistance of one ohm for one second  ( J , joule , watt second )
   a unit of electrical energy equal to the work done when a current of one ampere passes through a resistance of one ohm for one second