Definitions of girl:

• noun:   a young woman
• noun:   a friendly informal reference to a grown woman

  Example: "Mrs. Smith was just one of the girls"

• noun:   a female human offspring
• noun:   a youthful female person

  Example: "The baby was a girl"

• noun:   a girl or young woman with whom a man is romantically involved

  Example: "His girlfriend kicked him out"