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
  la jirafa - giraffe
  el karateka, la karateka - karate expert, someone who practices...
  Jehová, el Jehová - Jehovah or Yahweh (name of God often...
  la ictericia - jaundice
  el iceberg - iceberg
  integral - complete, comprehensive, total, made...
  el invierno - winter, rainy season (in the tropics)