Maiden is an old-fashioned term a girl or an unmarried young woman. If you like reading fantasy novels, you've probably encountered plenty of knights and maidens — and maybe even a dragon or two.
These days, you will hardly ever see this word used to mean an unmarried woman, except maybe humorously. However, you will see it used when discussing the maiden voyage of a ship or plane: in both cases, the word indicates youth and inexperience. Also, the maiden name of a married woman is her name before marriage, a name many women keep after marriage too. There is also a meaning of maiden in the baseball-like British game of cricket.