From Merriam Webster:

Noun...

• 1.
  • a question raised for inquiry, consideration, or solution
  • a proposition in mathematics or physics stating something to be done
• 2.
  • an intricate unsettled question
  • a source of perplexity, distress, or vexation
  • difficulty in understanding or accepting

I particularly like that first definition: a question raised for inquiry, consideration, or solution