sine qua non Look up sine qua non at Dictionary.com
"an indispensible condition," c.1600, from L., lit. "without which not," from sine "without" + qua ablative fem. sing. of qui "which" + non "not." Fem. to agree with implied causa. The L. phrase is common in Scholastic use. Sometimes a masc. form, sine quo non, is used when a person is intended. Proper plural is sine quibus non.