el kilometraje - mileage, but using kilometers rather...
  justo - just, fair (in the sense of being...
  el judas - traitor, betrayer
  el juguete - toy, plaything
  el jamón - ham
  la jabalina - javelin
  la jarra - jug, pitcher, mug (as for beer)
  el jefe, la jefa - chief, boss, manager, leader, head...
  el kayac - kayak, kayaking
  juvenil - youthful, youth-oriented, juvenile
  justificable - justifiable
  la jerarquía - hierarchy, someone high in the hierarchy
  la juntura - joint, junction, juncture, place...
  la jungla - jungle
  jaque, jaque mate - check, checkmate (chess terms)
  el jarabe - syrup
  la jalea - jelly, gelatin dessert
  jaibón - pretentious, snobbish, stuck-up,...
  el jengibre - ginger
  la jeringuilla - syringe