Indo-European languages are a large family of languages that are spoken in Europe, the Middle East, and parts of Asia. The most widely spoken Indo-European language is English, but there are many others, such as French, Spanish, German, Russian, and Hindi. The term 'Indo-European' is derived from the Latin word 'indus, ' meaning 'India.' The four-letter solution to the crossword puzzle is 'INDO, ' which is an abbreviation for the Indo-European language family.