The term 'legally sound' can be defined as having a VALID legal basis. The crossword solution for this term is 'VALID', which has five letters. Legally sound means that the law is being followed and that any action taken is within the bounds of the law. It also implies that any agreement or contract is legally binding and enforceable. Legally sound decisions are those that are made with the best interests of all parties in mind, and that are in accordance with the law.