noun

• (sports, slang) A referee.
• (medicine, slang) An unlikely diagnosis, especially for symptoms probably caused by a common ailment.
  ""It's a zebra! George. OK, I should explain. A zebra is..." "Medical slang for coming to an exotic diagnosis when a more simple explanation is more likely." "That's right. I was convinced that George, given his age and symptoms, had some kind of cardiac issue. It fit, it made sense. Because I was looking for the obvious when I should have been looking for the zebra! George is just having an allergic reaction to a combination of chemicals from all the stains and paints he's been using in the garage.""
• (medicine, by extension) Someone who has Ehlers-Danlos syndrome or hypermobility spectrum disorder
  "EDS charities around the world use a zebra logo to promote the idea that sometimes it really is that ‘rare’ condition."
• (derogatory, slang, ethnic slur) A biracial person, specifically one born to a Sub-Saharan African person and a white person.
  "“People change countries for all kinds of reasons,” Ross tells me. “But at least one of them was that she had this light-skinned, mixed-race child who had already been called a zebra at school.”"

noun

• (World War II era, joint US/RAF) radiotelephony clear-code word for the letter Z.