General terms:
* Brain teaser: This is a general term for any puzzle that requires thinking outside the box.
* Conundrum: This term implies a puzzle that is difficult to solve or understand.
* Enigma: This word suggests a puzzle that is mysterious and puzzling.
* Stumper: This refers to a puzzle that is particularly difficult to solve.
* Mind-bender: This suggests a puzzle that is intellectually challenging and requires creative thinking.
More specific terms:
* Cryptic crossword: This type of crossword puzzle features clues that are indirect and require ingenuity to decipher.
* Rubik's Cube: This classic puzzle is notoriously difficult to solve.
* Sudoku: This number puzzle can be incredibly challenging, especially at higher difficulty levels.
* Logic puzzle: This broad category encompasses a variety of puzzles that require logical deduction.
* Escape room: These real-life puzzles involve solving riddles and challenges to escape a locked room.
Figurative terms:
* A knotty problem: This metaphor suggests a puzzle that is tangled and difficult to unravel.
* A Gordian knot: This refers to a complex problem that seems impossible to solve.
* A Herculean task: This suggests a puzzle that is incredibly difficult and requires immense effort.
The best term to use depends on the specific context and the type of puzzle in question.