sine qua non Look up sine qua non at Dictionary.com
"an indispensable condition," Latin, literally "without which not," from sine "without" + qua ablative fem. singular of qui "which" + non "not." Feminine to agree with implied causa. The Latin phrase is common in Scholastic use. Sometimes a masculine form, sine quo non, is used when a person is intended. Proper plural is sine quibus non.