An enigma is an uncommon and particular surprising event or occurrence. Someone who normally shows up on time for work showing up two hours late without good reason is an enigma. Enigmatic behavior might cause someone to say, "He doesn't normally behave like this," to note the peculiarity.
Another meaning of enigma is a statement or picture with a hidden meaning, like a riddle. The term comes from 16th-century Latin and Greek words meaning to speak in riddles. Some movies or books end without a definitive conclusion or clear picture of what has happened. This enigma causes readers or viewers to ponder the culmination of the story or its meaning.