An enigma is a puzzle, or something mysterious and difficult to explain. People often use the term to imply that the puzzle or problem is intellectual in nature, requiring the full force of someone's deductive ability to solve it. The term was borrowed as a name for a coding machine used in the Second World War; the Enigma machine relied on a complex series of steps and devices to make virtually unbreakable codes. The only way to solve them was to steal a machine and its codebook, since the codes it generated were so complex.
The term is derived from the Greek ainig, which means "to speak in riddles." Enigmas can take a wide range of forms, including puzzles, riddles, mysteries, secrets, ambiguities, conundrums, perplexities, and things which are simply inexplicable. They may also combine these elements, as Winston Churchill famously noted when he said that "Russia is a riddle wrapped inside a mystery inside an enigma."
Humans have a natural sense of curiosity, so puzzles are often a topic of intense interest, discussion, and debate before they are solved. Some famous historical enigmas have captivated the imagination of some of the greatest minds in the world, and continue to do so, as is the case with Fermat's last theorem in mathematics. Some people relish the intellectual challenge of such a mystery, as it allows them to prove their intellectual capabilities to themselves and others.
Some employers use puzzles in interviews or employment testing to determine whether employees are a good fit. These challenges may relate to the field the company works in, as might be the case with a software company that challenges applicants to find the flaw in a presented piece of code, or they might just be general intellectual challenges. The successful completion of a puzzle can indicate that an employee will be able to think creatively to solve problems, and the process that the applicant goes through can also be of interest to potential employers.
Someone who speaks in riddles may be called "enigmatic." Many examples of such individuals can be found in history and in literature, and these people are sometimes treated as great minds, since it is assumed that their brains simply work too quickly to allow normal individuals to comprehend them.