A problem that is challenging to solve.
Used other than figuratively or idiomatically: see hard, nut, crack.
A situation, person, group, etc. which is difficult to overcome or deal with.
A place, opportunity, etc. to which it is difficult to gain entry.
An amount that is difficult to finance.
A puzzling situation or problem; an enigma.
A person who solves puzzles as a hobby.
A video game in which the player is presented with (usually abstract) puzzles to solve.