noun

• (historical) A two-cornered hat, worn by European and American military and naval officers from the 1790s.
  "Just last Sunday, one of the estimated 20 surviving bicorne hats worn by Napoleon was sold at auction for €1.9mn, a price that vindicates the famous epigram, attributed to the emperor among others, that there is but one step separating the sublime from the ridiculous."
• (mathematics) A plane curve having two cusps.
• (rare) A beast having two horns, either real or fictional.
  "What a stir about nothing! Phenicopter is no more (pace Dr Septimus Wind and his eyriecal friend, Dr Eagles) than the common flamingo (simplex). Why ask such silly questions? What, for example, is a caterpillar? and why so denominated? There is the common quail as there is the common flag-beetle. A king (or queen) fisher might devour eaglets, but what is that to the spider in ordinary? and what has the spider to do with the Neapolitan tarentella? Unicorns and bicorns may be gregarious without even subterfuge or sideral ornament."