An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) A problem that cannot be easily or readily solved.
2) A game, question, or problem designed for testing ingenuity.

Of unknown origin.

Cistân from cist "what is?," + -ân noun suffix.

The question of the origin and nature of the → ultra high energy cosmic rays. According to the → GZK cutoff, the UHECRs should be nearby. They are expected to be exceptional, therefore visible by some astrophysical counterpart. However, there is nothing visible (within a few tens of → Mpc) in the direction of all the UHECR detected up to now.

→ UHECR; → puzzle.