The term 'probandi' is derived from Latin and refers to a person who is the subject of a legal or medical investigation. The crossword solution for this term is 'ONUS', which is a Latin word meaning 'burden'. This term is often used in legal contexts to refer to the burden of proof that must be met by a party in order to prove their case. In medical contexts, it is used to refer to the responsibility of a patient to provide information about their medical history and condition.