Is it Honus or Onus?

Excuse me, but could you clarify for me which spelling is correct: is it "Honus" or "Onus"? I've seen both versions used in different contexts and I'm having trouble determining which one is appropriate for the situation I'm currently dealing with. Could you possibly provide some insight into the origins and meanings of each word, as well as some examples of how they are typically used in sentences? Thank you in advance for your assistance.