The term SINN can be solved by two crossword solutions, FEIN (4 letters) and FIN (3 letters). FEIN stands for Federal Employer Identification Number, which is a unique nine-digit number assigned to businesses in the United States by the Internal Revenue Service. FIN stands for Financial Identification Number, which is an international standard for identifying financial institutions. Both numbers are used to identify and track businesses and financial institutions for tax and other regulatory purposes.