'brain-teaser' definitions:

Definition of 'brain-teaser'

(from WordNet)
noun
A difficult problem [syn: riddle, conundrum, enigma, brain-teaser]

Definition of 'brain-teaser'

From: GCIDE
  • brain-teaser \brain-teaser\ n. a difficult problem.
  • Syn: riddle, conundrum, enigma. [WordNet 1.5]