The term 'Mother of Mankind' is a reference to the biblical figure EVE, who is traditionally seen as the first woman and mother of all humanity. In the Bible, EVE is said to have been created by God from the rib of Adam, the first man. She is thus seen as the mother of all mankind, and her name is often used to refer to this concept. The crossword solution for this term is 'EVE', which is a three-letter word.